Kyle Bartley scored City's winner in the second minute of extra-time to see his side through to the second round of the competition and Clark was quick to praise him and his team-mates.
"They are a great bunch of lads and we have a great spirit. They want to learn and if they continue to do that I think we could do a lot more," he said.
Clark was not happy with some aspects of his side's performance though, especially the way they allowed Plymouth into the game.
"To give away two goals indicated that something was not right. Fortunately we had enough in the locker to win," he added.
"To give Plymouth the opportunities was the disappointing thing. It was a little frustrating and something the players will have to work on very quickly.
"We scored some great goals but I told the players at half-time that the name of the game is scoring goals and that there could be a shock result.
"They gave us a problem as they took us to extra time but it was a fantastic winning goal from Kyle Bartley.
"The problem for me is that I sometimes forget how young some of the players are because they are inexperienced at this level but they have to adapt quickly and change their mentality."
Plymouth manager John Sheridan admitted he was disappointed with the result but encouraged by his side's performance.
"We have just got to carry on improving. We showed against Birmingham just how well we can play and I think we will be at the right end of the division," he said.
"The performance was a big plus for me. The players responded really well.
"To come back from behind on two occasions showed character and that was particularly pleasing for me."