"I'm absolutely delighted. I think to win a game like that against the quality of opposition we're up against is very satisfying," BBC Sport quotes Clark as saying.
"Not only are Swansea the holders but I said before the game that I felt it was the hardest tie we could have got outside the so-called top six, a club that's on the up in everything that they do.
"Even though they made changes, I don't think it affects them as they've got a philosophy that runs through the club."
Wilfired Bony netted a consolation goal for Swansea in the 90th minute.