The 39-year-old, who is a manager with TSB Bank, sparked his sensational effort with a burst of four straight birdies from the third before adding further gains at the 10th, 11th, 15th, 17th and 18th as he kept his foot to the pedal right to the finish.
“I played great all day,” said Clark, a sterling servant to Scotland over the years in the amateur ranks. “It helped having Jamie (Mann) and Luke (Harries) alongside me also playing well.
“I hit a lot of good golf shots, played conservatively and avoided the bunkers, which you need to do around this golf course.
“While I’ve played in the championship enough times, I’ve never qualified for the match-play before, but my game was trending, I think is the current word.”
Having opened with a 72 in tougher conditions on Monday, Clark finished with a nine-under total, pipping England’s Jack Dyer (67) by a shot, with Suffolk-based Scot Gregor Tait and Ireland’s Gregor Tait next on three-under.
“It’s a nice place to get a course record, but I think you’d give that and the 62 up to hold the trophy on Saturday,” said Clark.
“That’s what we’ve all come here for and they’ll all be gunning for the guy who did something good today, so let’s see how it goes.”
Tait, a 23-year-old who plays at Aldeburgh, signed for six birdies as he comfortably made the top 64 at the first attempt.
“My whole family is Scottish,” he said. “My dad grew up in Bathgate and my mum was born in Aberdeen and, though the accent doesn’t suggest as much, I have always been a proud Scot.”
Host club member Calum Scott, who had led after an opening 67, also progressed as one of the leading seeds after adding a par-72 that was bookended with birdies.
“I’ve done well in match-play in the past and hopefully I can have an advantage at my home club,” he said. “I know when to be aggressive and when to hold back.”