The visitors twice came from behind to level, firstly through a Marlon King penalty and then a Peter Lovenkrands strike with eight minutes left, but immediately conceded a Derby winner to Nathan Tyson.
Clark said in the aftermath: "I thought our second half performance warranted something but our first-half performance didn't. It's a good summary of our season really, it's a Jekyll and Hyde performance in one game.
"It's been like that game to game for us. We've played well in one game and the next it doesn't look the same team but we had it in one game tonight.
"The first half wasn't our true identity but the second half was. I said to the players at the end that if we play like that from the first whistle and don't allow the opposition to get momentum and get on the front foot, we will get a result.
"I don't want to make excuses but facts are facts. The first goal was offside, that's a fact and the second goal, Marlon King has been fouled on the edge of the box. I've seen the incidents again and that's what happened."