It's a container, however, for an old instrument (7)
I believe the answer is:
sackbut
'an old instrument' is the definition.
(I've seen this before)
'it's a container however' is the wordplay.
'it's a container' becomes 'sack' (sack is a kind of container).
'however' becomes 'but' (I've seen this before).
'sack'+'but'='SACKBUT'
'for' is the link.
(Other definitions for sackbut that I've seen before include "Early form of trombone" , "music maker" , "music-maker" , "This was in the wind" , "Old trombone" .)