Boy finds key for tune (4)
I believe the answer is:
song
'tune' is the definition.
(thesaurus)
'boy finds key' is the wordplay.
'boy' becomes 'son' ('son' can be a synonym of 'boy').
'finds' is a charade indicator (letters next to each other).
'key' becomes 'g' (I've seen this in other clues**).
'son'+'g'='SONG'
'for' is the link.
(Other definitions for song that I've seen before include "Musical composition; air" , "Musical rendition" , "Lied" , "Vocal number" , "Tune" .)