The young defender is on-loan from Swansea City.
Clark, asked if Bartley had been living up to potential and punching his weight this season, responded: "Certainly in the last couple of games. In spells he has.
"I still think there's a lot more to come from him because you see the size of him, the athleticism.
"He has got a change of pace, which I have told him before, that not many defenders outside the Premier League have.
"And, when it is required, he can pass as well being very comfortable on the ball.
"He has done really well but I do think there is a lot more to come from him."
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