Not many in audition — what a relief! (4)
I believe the answer is:
phew
'what a relief' is the definition.
(I've seen this in other clues)
'not many in audition' is the wordplay.
'not many' becomes 'few' ('few' is the opposite to 'many').
'in audition' shows a homophone (sound like) (audition can mean hearing).
'few' is a homophone of 'PHEW'.
(Other definitions for phew that I've seen before include "Sound relieved" , "Exclamation of relief" , "Gosh, that's a relief!" , "Expression of relief" , "thank goodness!" .)