Container with a musical instrument (4)
I believe the answer is:
tuba
'musical instrument' is the definition.
(I know that tuba is a type of musical instrument)
'container with a' is the wordplay.
'container' becomes 'tub' (tub is a kind of container).
'with' says to put letters next to each other.
'tub'+'a'='TUBA'
(Other definitions for tuba that I've seen before include "item band might play" , "music-maker" , "Player" , "blow it!" , "Big brass instrument" .)