Boy finds key for tune (4)

I believe the answer is:
song
'tune' is the definition.
(I've seen this in another clue)
'boy finds key' is the wordplay.
I cannot quite see how this works, but
'boy' could be 'son' (male child) and 'son' is found within the answer.
'key' could be 'g' (musical key) and 'g' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'for' acts as a link.
Can you help me to learn more?
(Other definitions for song that I've seen before include "Musical composition; air" , "Musical rendition" , "Lied" , "Vocal number" , "Tune" .)
