"That's painful!" (4)
I believe the answer is:
ouch
'that's painful' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ouch that I've seen before include "response to a bad pun" , "that's painful" , "sounds painful" , "Exclamation of pain - that hurt!" , "Exclamation of sudden pain" .)