The Baggies were in cruise control after Romelu Lukaku had put them 2-0 up but Reading scored three goals in the last nine minutes to snatch victory from the jaws of defeat.
Jimmy Kebe stole in at the back post to beat Ben Foster with a header before Adam Le Fondre scored a penalty and Pavel Pogrebnyak bagged the winner in dramatic late turnaround.
"We had good control and I certainly couldn't see the finish to the game that happened," said Clark, who denied it was down to complacency or a sense that his players felt the game was won. "I am disappointed. I am not sure (what happened). I can't be more honest than that.
"We should have scored four or five but then we lost a goal and for some reason we lost our shape and we lost our composure and conceded the two late goals that cost us the game.
"When you contrive to pull a defeat from what should be a comfortable victory then it is something we will have to look at. Football can be a strange game and this is one of those days."
Clarke was not convinced that Jonas Olsson's tackle on Kebe warranted a penalty.
He said: "It is clumsy but I watched it four or five times and I have a bit of a doubt."
The result overshadowed an impressive performance from Lukaku.
"I said he would improve this season and he is improving. But he is as disappointed as anybody else. Two goals don't mean anything when you get beaten," Clarke added.
- Sports & Recreation
- Ice Hockey
- Jimmy Kebe
- Romelu Lukaku
- Adam Le Fondre