The Northern Irishman, without a tournament victory since his Open triumph at Sandwich two and a half years ago, shrugged off the blustery conditions to fire five birdies and two bogeys to finish two strokes behind leader Raphael Jacquelin of France.
Clarke, ranked 297th in the world, attributed his good start at the first European Tour event of the year to the fact he has lost almost 20kg in weight in recent months.
"I got to the autumn and looked at myself and the word fat sprang to mind so I decided to do something about it," the 45-year-old told reporters at the Durban Country Club.
"I have a trainer in Dublin who shouts at me and I shout back and it seems to be working. It's a regime change but not too drastic, I still squeezed in a pint or two over Christmas."
Clarke said he had largely eliminated sugar and carbohydrates from his diet but quickly joked: "I have not gone totally over to the dark side".