The 6ft 8in Serbian striker, axed for Saturday's 4-0 defeat by Watford following his lacklustre efforts in training, put in a busy shift despite missing more than one opportunity.
"Nikola's response was fantastic, absolutely brilliant," Clark said.
"I spoke with him on Monday morning and the two things I said were, I think he's an extremely talented footballer, and I've got a lot of respect for him as a guy.
"But he has to understand I've got my principles, my standards, my rules. As I said, there was no rant, no shouting, it was me just making a decision I felt was right.
"It's gone now. He's responded in the right way, he's put in a terrific 94 minutes for us - work ethic, quality on the ball - I'm just disappointed for him he didn't get the goal his performance probably deserved.
"He showed me by his performance he hears what I'm saying and he's gone about his business in the right and proper way.
"He's showed he's not just big in size, he's a big man as well."