Newly-promoted Charlton appeared to be heading for a smart away win after an 81st minute goal from Leon Cort but substitute Zigic swooped to score, almost casually, a face saving goal with a low shot.
It may prove a goodbye gesture with Birmingham reported to be in the process of unloading the lanky striker although Clark claims there is currently no interest in the player.
Clark admitted: "It was not our most fluent performance. Some of our players were not exploding. These things happen in the early part of the season.
"It is credit to them that they showed great character to come back from what you were thinking was going to be a defeat when you are in injury time and a goal down to snatch a point.
"It is sign of a good time to come back as they did when our general performance has not been great."
Clark gave particular credit to Zigic, saying: "He is a top quality player. His attitude and application since I've been at the club has been sensational.
"I know there is speculation but I have spoken to him and he likes the club and the area.
"Scoring this equaliser has given me a little nudge that he can be a major asset.
"He made it difficult for the Charlton back line and scored a magnificent goal."
On his Football League debut for Birmingham, just three days after making his first appearance for England, teenaged goalkeeper Jack Butland had a quiet game as he was rarely pressured by the Charlton attack.
England's youngest ever goalkeeper did not have to make his first serious save until the 42nd minute when he confidently handled a Yann Kermorgant shot but he had little chance when Cort opened the scoring with a close range header.
Zigic was a late substitute as Birmingham tried to salvage a point, and he responded with the equaliser from Chris Burke's pass to perhaps put himself back in Birmingham's plans.
Charlton boss Chris Powell said: "We came to a big ground where you really have to earn the right to win a point.
"On one hand I am disappointed we dropped two points after getting so close to a win.
"But I have to give my team a lot of plaudits for the performance. Obviously we went so close but before the game I think people would have said to get a point at Birmingham was a real positive display.
"But you have to see a game right to the end. That is why Zigic cost ?6m. He is quality and he showed that with his equaliser.
"We were on top in the last 20 minutes when Birmingham were on the back foot. This is why we needed to stay on top right until the end. But we didn't see the game through to the end.
"But saying that it was an extremely well worked goal. The players cannot be faulted from what I've seen from the television replay.
"What I do like about the game is that from the first minute we were always in the game. We didn't sit back and be starry eyed because we were at St. Andrew's.
"We took the game to Birmingham and should have come away with a win. Another day we might not play so well and win. But it is a real confidence boost for the players.
"It gives them a belief in themselves that they can perform in the Championship."