Key in container (3)
I believe the answer is:
bin
'container' is the definition.
(bin is a kind of container)
'key in' is the wordplay.
'key' becomes 'b' (eg B major, musical key**).
'b'+'in'='BIN'
(Other definitions for bin that I've seen before include "Discard" , "Graves seen here" , "Storage container" , "Receptacle for kitchen rubbish" , "Where to put rubbish" .)