"What fun!" (4)
I believe the answer is:
whee
'what fun' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for whee that I've seen before include "Expression of delight" , "Cry from someone going downhill fast?" , "Exclamation of excitement when going fast" , "What fun" , "Exuberant cry" .)