Cockney tramp overheard, one found among the reeds? (4)
I believe the answer is:
'one found among the reeds?' is the definition.
'cockney tramp overheard' is the wordplay.
'cockney tramp' becomes ''obo' ('hobo' - synonyms - said with the 'H' dropped - a cockney accent.).
'overheard' becomes 'e' (I am not sure about this - if you are sure you should believe this answer much more).
(Other definitions for oboe that I've seen before include "issuer of notes" , "Double-reed wind instrument" , "Woodwind instrument of treble pitch" , "Orchestral piece" , "Double-reed woodwind" .)