The 1-0 defeat left the Blues languishing at the wrong end of the table, just five points above the drop zone.
Cardiff secured the spoils thanks to the only goal of the game coming from a blunder by keeper Jack Butland, who parried a wayward Craig Conway shot into the path of scorer Joe Mason.
"The goal we gave away was very unfortunate," Clark said. "Butland saves the shot but then the ball hit him on the back of the head and then on the back of the heel and rolls straight to Joe Mason to tap it in.
"We played some excellent football. It is horrible because we are in a business where picking up is vitally important but all the performances collectively was very good against a team which is going to get promoted.
"The only point I was made to the players was that in the second half we didn't pull the trigger as we tried to walk the ball into the net.
"I thought Nathan Redmond and Ravel Morrison made a good job of a difficult situation playing up front. But it is difficult when they do not have the natural instinct of a striker.
"They are tricky players who want to take someone on and bend the ball into the top corner of the net. Sometimes when you have that natural instinct as a striker you pull the trigger."