Alex Kacaniklic's 58th-minute goal settled matters at The Hawthorns after on-loan striker Romelu Lukaku had drawn West Brom level following Dimitar Berbatov's opener for Fulham.
The defeat means the Baggies won two and lost two over the festive period, but Clarke did not want to blame the busy schedule for a third home loss of the season.
"I thought we were poor in the first half, especially," Clarke told Sky Sports. "It looked like a fourth game in a short space of time.
"We looked a little bit jaded and didn't play with our normal intensity, but in the second half we were much better and got about the game.
"We got back into the game and were creating chances, but got caught by a sucker-punch. It's difficult enough coming back from 1-0 down to get back into the game, but to ask to come from 2-1 down was a little bit too much for us."
Asked if the heavy schedule affected West Brom's performance, Clarke replied: "I don't think so, that would just be looking for excuses.
"We have to look at ourselves and we didn't play as well as we can.
"We've had a great run at home and have always started games on the front foot and made it difficult for teams, but we didn't do enough of that."