Clark was caught in possession by Edin Dzeko when trying to be over-elaborate and appealed in vain for a foul as he threaded the ball through to Tevez, who kept calm and cut back inside the box before slotting past Brad Guzan.
Lambert said: "I thought we were well in the game. The goal is a mistake. I wouldn't have minded if someone had put one in the top corner with a bit of skill.
"But it's a mistake by us. It is easy to be critical of Ciaran. The lad has been excellent all season. He will learn from it, he won't make the same mistake again."
Midfielder Fabian Delph will be suspended for the next two games against Reading and QPR after collecting his 10th booking of the season for handball.
Lambert said: "He's playing extremely well, really well. You are pressing forward, to try and get a goal back and things like that happen."
The Villa boss knows the significance of the next two matches with his team still sat in the relegation zone.
He said: "The next two games are massive for us. If we play like that, we will give teams a fright, that's for sure."