Robert Snodgrass gave the Canaries a half-time lead, a Ben Foster howler gifted Grant Holt a second, the third was an own goal from Gareth McAuley and Jonny Howson scored a spectacular late fourth to leave Clarke to reflect on a forgettable afternoon.
"We've been terrific this season, here we were the opposite," he said. "We were really poor.
"It's a blow to us because that's not a normal performance for West Bromwich Albion. We were nowhere near it. It was Norwich's day, not ours."
Foster's decisive error with 28 minutes remaining gave the Canaries some breathing space.
The England goalkeeper met Wes Hoolahan's prod forward with a mis-kick which allowed Holt to walk the ball in. But his manager refused to criticise Foster.
"It's a mistake," Clarke said. "He knows it's a mistake. He didn't do it on purpose. And it was pivotal in the game.
"At 1-0 there was still the nervousness and the tension around the stadium. At 2-0 it released the pressure and they ran away with the game at the end."
Peter Odemwingie came on as a substitute late on after a second half of the season in which his Twitter tirades have been an unwanted distraction and Clarke expects the Nigeria forward to leave this summer.
The West Brom boss said: "His future will be decided in the summer. I don't think he'll be here next season."
- Sports & Recreation
- West Bromwich Albion
- Jonny Howson
- Gareth McAuley
- Robert Snodgrass
- Grant Holt