Found key in box (6)
I believe the answer is:
create
'found' is the definition.
(to found is to establish or create)
'key in box' is the wordplay.
'key' becomes 'e' (musical key).
'in' indicates putting letters inside.
'box' becomes 'crate' (crate is a kind of box).
'e' placed within 'crate' is 'CREATE'.
(Other definitions for create that I've seen before include "Build" , "Establish" , "coin" , "Bring into existence" , "Compose" .)