Sudden disappointment (4)
I believe the answer is:
blow
'sudden disappointment' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for blow that I've seen before include "Strong current of air" , "Whistle - hit" , "'..., ..., thou winter wind' (As You Like It)" , "cocaine from US" , "Sudden disappointment" .)