"How clumsy of me" (4)
I believe the answer is:
oops
'how clumsy of me' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for oops that I've seen before include "I've messed up" , "Silly me, I've dropped something!" , "Exclamation of making a blunder" , "I'm sorry" , "Exclamation on making minor error" .)