Not very many heard — what a relief! (4)
I believe the answer is:
phew
'what a relief' is the definition.
(I've seen this in other clues)
'not very many heard' is the wordplay.
'not very many' becomes 'few' (I've seen this in another clue).
'heard' indicates a 'sounds like' (homophone) clue.
'few' sounds like 'PHEW'.
(Other definitions for phew that I've seen before include "Gosh, that's a relief!" , "Sound relieved" , "Exclamation of relief" , "Good gracious, its hot!" , "Recover unharmed" .)