Returned neighbour's instrument (4)
I believe the answer is:
tuba
'instrument' is the definition.
(tuba is a kind of instrument)
'returned neighbour's' is the wordplay.
'returned' shows that the letters should be reversed in order.
'neighbour' becomes 'abut' (I've seen this before).
'abut' written backwards gives 'TUBA'.
(Other definitions for tuba that I've seen before include "Brass band instrument" , "Large low-pitched brass musical instrument" , "Big brass instrument" , "part of 14 [ORCHESTRA] ?" , "music-maker" .)