Returning neighbour’s musical instrument (4)
I believe the answer is:
tuba
'musical instrument' is the definition.
(tuba is a kind of musical instrument)
'returning neighbour's' is the wordplay.
'returning' is a reversal indicator.
'neighbour' becomes 'abut' (I've seen this before).
'abut' back-to-front is 'TUBA'.
(Other definitions for tuba that I've seen before include "Producer of note" , "Brassy thing" , "member of band" , "bit of brass" , "Stop" .)