Zigic, 31, had been strongly linked with a move away from the Blues in the transfer window, but a proposed transfer to Spain fell through at the final moment.
"I have got the utmost respect for the guy. He's a good man, he's a talented footballer. I just wanted it resolved one way or the other and now it has been cleared up we can all get on with things," Clark told the Birmingham Mail.
"His focus in training has been fantastic. Whether the speculation has affected him a little bit, only Ziggy can answer that.
"But he's certainly looked a lot more at ease around the training ground, around the place, and he's been very impressive. Nikola Zigic firing on all cylinders is a major asset for anyone - and that's what we want."
Birmingham currently sit 21st in the Championship table.