Join fool for audition (4)
I believe the answer is:
knit
'join' is the definition.
(knitting is a kind of joining)
'fool for audition' is the wordplay.
'fool' becomes 'nit' (both can mean a stupid person).
'for audition' shows a homophone (sound like) (audition can mean hearing).
'nit' sounds like 'KNIT'.
(Other definitions for knit that I've seen before include "Draw (brows) together" , "Work with wool and needles" , "(Of broken bone) unite" , "Mend" , "Wrinkle" .)