Essex 126 (Clarke 4-28, Westley 49) and 13 for 0 trail Surrey 351 (Clarke 56) by 212 runs
Surrey took another significant step on their way towards a first Championship title since 2002 as they bowled Essex out for 126 and enforced the follow-on at Chelmsford.
With Somerset's game against Lancashire ending in a spectacular tie as they failed to chase 78 on a turning pitch at Taunton, Surrey will go 43 points clear at the top if they complete victory here, and if this performance from their bowlers is anything to go by, they may not even have to bat again.
Their four-pronged seam attack of Morne Morkel, Tom Curran, Rikki Clarke and Conor McKerr shared ten wickets between them, and it was Clarke who did most of the damage. In a nine-over spell from the River End he removed Ravi Bopara, Ryan ten Doeschate, Tom Westley and Simon Harmer to quash any hopes of an Essex recovery after they had slumped to 20 for 3.
Clarke moved the ball considerably in the air, finding late swing and benefiting from the variable bounce on offer - a handful deliveries shot up off the pitch - as he rattled through the middle-order in conditions that could hardly have suited his medium-fast seamers better; the sun did not so much as threaten to escape behind a sheet of clouds, and bad light and drizzle brought proceedings to an end only seven overs after tea.
As much as Surrey's season will be remembered for Rory Burns' runs and Morkel's fearsome mid-season form, it is impossible to overlook Clarke's contribution. He now has 37 Championship wickets at a shade over 20, to go with 392 runs and 13 catches; he turns 37 at the end of this season, and not even his most ardent supporters could have predicted the transformative effect he would have on this side when his move from Warwickshire was confirmed last season.
Clarke also added useful runs in the morning session in an enterprising innings of 56. After pulling his first ball for a nonchalant four, he offered stubborn resistance in bowler-friendly conditions alongside Curran, before clubbing 26 runs off his final 13 balls as he ran out of partners.
It was a largely brutish innings, with a couple of orthodox slogs down the ground for six, but he threw in a deft reverse-sweep and an elegant cut as if only to prove how malleable his game is.
While Essex will attribute their surrender to poor shots and a batting order that has not fired all season, Surrey were irresistible with the ball. Curran and Morkel were hostile in their opening spells, as their accounted for Nick Browne, Varun Chopra, and Dan Lawrence, before Clarke ran through the middle order and McKerr mopped up the tail.
On the day that Alastair Cook's new three-year deal at the club was confirmed, Chopra must have felt disappointed with his dismissal, caught well by (guess who?) Clarke in the slips playing a flashy drive at a ball he had no need to play. He now averages 16.7 this season; when Cook returns, he looks like the man most likely to drop out.
Tom Westley, another Essex batsman struggling for runs, was the only one in the top eight to reach double figures, and fought hard before falling one short of just a second half-century of the season.
Westley was not at his fluent best - as was the case in his gritty 56 at Taunton and in last week's win over Hampshire - but he was organised, determined, and creamed a pair of cover drives for four in the space of three Curran balls. He looks to have benefitted from the decision to leave him out of the second half of Essex's Blast campaign, and will hope that he can find his best form before the season's close.
The tag of champions-elect can loom over a team at this stage of the season, but Surrey look good value for it. Their visit to Taunton in the penultimate round of games had been highlighted as a possible title-decider - and they can expect another lively wicket if the Championship race is still alive - but wins here and at New Road next week would effectively wrap the pennant up.
On the basis of this performance, it would be impossible to argue that they have not been good value for it.