Chris Turner's side, unbeaten in five games before the match, were leading 1-0 thanks to a Gary Liddle header but succumbed to constant Huddersfield pressure.
A Peter Hartley own goal and a stunning solo effort from Williams earned Town a fifth straight win and preserved their unbeaten home record.
Clark said: "It was a terrific goal and there's no doubt that Robbie possesses many attributes as an attacking full-back.
"But I've laid the gauntlet down to him that he has to improve on his defending. He knows that with the competition we have for places that if he doesn't improve and keep up the consistency then he could be out of the side.
"I thought we deserved to win. Apart from their goal and the one they had cleared off the line it took until the 65th minute for them to get a shot on target. But there's no doubt this was one of the most satisfying wins of the season because we had to come back from a goal down."
