The home side established a 3-0 lead as the Baggies struggled to get into the contest and despite Romelu Lukaku pulling one back before the break, they were unable to avoid defeat.
Victory would have been the first time West Brom had managed five top-flight victories in a row for 34 years but Clarke admitted they were always struggling to keep pace with their opponents.
"That was very unexpected. We started so slowly it was incredible, you wouldn't believe it," said the Scotsman.
"We know Swansea are a fantastic team and they play good football and we knew they would cause us problems.
"But I did not expect so many problems so early and when you are chasing the game it is going to be difficult.
"We have had a really good season, played really well. We came into the game confident, with more or less the same players and system, but that's football, it happens.
"They had a quick start, they got in front and we left ourselves too much to do to be honest."
Meanwhile, Clarke revealed that goalkeeper Ben Foster is in Germany and will undergo a second operation on his groin injury.
"You would have to ask the doctor but he is in Germany and has either had the surgery, or is going to have it," added Clarke.