Expression of relief (4)
I believe the answer is:
phew
'expression of relief' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for phew that I've seen before include "Gosh, I'm sweltering!" , "Gosh, that's a relief!" , "Exclamation of relief" , "What a relief! It's hot!" , "Expression of relief" .)