Freddie Heldreich impressed again after Emilio Gay went to a classy maiden T20 half century, but Durham held their nerve to go to a four-wicket win at Seat Unique Riverside.
Clark continued where he left off in the return fixture, his knock of 55 provided the foundation for the chase that was threatened by three wickets from Freddie Heldreich.
Ricardo Vasconcelos was ruled out by the same troublesome wrist that saw him miss the fixture against Leicestershire. His replacement Gay was the early pacesetter, impressing with his striking down the ground, including back-to-back boundaries against Luke Robinson. Chris Lynn made a slow start to his knock but brought up the fifty stand with a fine cut behind point to the fence.
After clearing the rope for the first time, the Australian was starting to find his range, only to fall for 24 with Liam Trevaskis claiming a low catch at long-off. Sowter was then responsible for the second wicket as Josh Cobb got himself in a mighty tangle after surviving an lbw shout and was run out by Ashton Turner.
Gay continued to play the anchor role and brought up his maiden T20 fifty from 38 balls to position his side for a late surge over the 150-run mark.
Sowter continued his form against the Steelbacks this season however as he prised out David Willey and bowled Saif Zaib to put the clamps on the visitors. The leg-spinner ended his spell with his fourth scalp as Gay was caught on the boundary for 53, taking his ninth wicket in two games against Northamptonshire for the cost of just 29 runs.
The Steelbacks scrambled to a competitive total of 161 for seven courtesy of useful late cameos from AJ Tye and Tom Taylor.
The Steelbacks missed a golden opportunity to set the Durham chase back in the second over when Taylor put down a chance at short midwicket to dismiss Alex Lees on 1. The Durham skipper and Clark surged past their fifty partnership at the end of the powerplay, moving ahead of the required rate.
Heldreich earned brief respite for the visitors by breaking the stand with his first ball, pinning Lees lbw for 30.
Clark followed in the footsteps of his team-mate Sowter by defying the Steelbacks again. After scoring his maiden T20 century at Wantage Road in the first match of the competition, he proved equally destructive on home soil, powering his way to fifty from 25 balls.
Heldreich kept the visitors in with a shout of hauling back the hosts working in tandem with a miserly spell from Saif Zaib, removing Michael Jones and the key wicket of Clark for 55. Zaib then cranked up the pressure when Ollie Robinson was caught on the fence, ending his four-over spell for one for 17.
Turner eased the tension for the hosts and all but carried them over the line with a vital knock of 31 from 18. Liam Trevaskis wrapped up proceedings by sending Tye over the rope in the penultimate over.