Cockney tramp heard instrument (4)
I believe the answer is:
oboe
'instrument' is the definition.
(type of musical instrument)
'cockney tramp heard' is the wordplay.
'cockney tramp' becomes ''obo' (Cockney pronunciation of 'hobo').
'heard' becomes 'e' (I've seen this in other clues).
'obo'+'e'='OBOE'
(Other definitions for oboe that I've seen before include "Source of music" , "Double-reeded woodwind instrument" , "blow it!" , "Slender double-reed instrument" , "Orchestral instrument" .)