The reigning South African Open champion carded a 6-under-par 66 around the Woodhill Country Club layout, joining American Bruce Vaughan, Ireland's David Higgins, Andrew McLardy and Michael Kirk one stroke clear of the field.
Graeme van der Nest stands alone on 5-under and himself a stroke clear of a group of seven professionals on 4-under-par.
The first round congestion was largely due to a tricky wind, which kept the field together.
But it was on the greens where Clark felt he was not totally in control of his round.
'I haven't been putting all that well,' he said.
'I thought my South African Open win sorted that out. But I think it's just a case of me not having played tournament golf for so long. I'm just not comfortable with the putter.'
Clark spent most of last year sidelined with a wrist injury and made his comeback in the South African Open.
Apart from his indifferent form with the putter, Clark was also not satisfied with his performance on the par-5s.
'I'm a bit disappointed that I only managed to play the par-5s in 1-under,' he said.
'But the greens were quite firm out there, and some of the pin positions were tough, especially on those holes played into the wind.'
Vaughan, who led this event after the second round last year, was also not entirely enthusiastic about his first round.
The avid hunter from the small Kansas town of Hutchinson has long not been impressed with this game, and he seemed shocked at his ability to post a competitive score.
'This is quite a turnaround from the way I've been playing,' said Vaughan, who missed the cut by one shot in both the South African Open and the Dunhill Championship.
'I haven't been making putts. That was the difference today (Thursday). The course is also not playing all that hard, and any holes which are played straight into the wind are quite short,' he said.
Local favourite Martin Maritz could not replicate the form which has rocketed him up to fifth place on the Sunshine Tour's money list the last few weeks.
Maritz battled his way to a 2-over-par 74, featuring back-to-back bogeys at holes 14 and 15.
Zimbabwean Mark McNulty signed for a 1-under par 71 on the first day, while his countryman Marc Cayeux had to settle for a 70.
Cayeux was at 5-under playing the last before suffering a triple bogey at the par-4 ninth.
The young Zimbabwean lost his tee shot, forcing him to tee off again at a hole which ended with a three-putt for a seven.
England's 20-year-old Sandeep Grewal, who was in contention for last week's Dunhill Championship, opened with a 1-under-par 71.