Albion have now won only two of their last 11 matches in all competitions.
Clarke said: "I was disappointed, I thought we had the better chances and Rob Green made some good saves but we went to sleep at a set play.
"The squad's been stretched in recent weeks, the boys have done their best. Maybe the luck we had a few weeks ago has deserted us.
"But as a group, we know what we are trying to do. There's no better team to try and turn it around than against Aston Villa at the weekend.
"There was no hangover from the weekend defeat at Reading tonight. We had good control of the game.
"We played some decent stuff without having a spark. It could not have been a great spectacle but we were the team trying to win it."