Cockney tramp heard musical instrument (4)
I believe the answer is:
oboe
'musical instrument' is the definition.
(oboe is a kind of musical instrument)
'cockney tramp heard' is the wordplay.
'cockney tramp' becomes ''obo' (Cockney pronunciation of 'hobo').
'heard' indicates a 'sounds like' (homophone) clue.
'obo' is a homophone of 'OBOE'.
(Other definitions for oboe that I've seen before include "Orchestral piece" , "Double-reed woodwind" , "source of notes" , "Double-reed wind instrument" , "Producer of notes" .)