Neighbour’s brass instrument returned (4)
I believe the answer is:
abut
'neighbour's' is the definition.
(I've seen this before)
'brass instrument returned' is the wordplay.
'brass instrument' becomes 'tuba' (I've seen this before).
'returned' is a reversal indicator.
'tuba' reversed gives 'ABUT'.
(Other definitions for abut that I've seen before include "Impinge" , "Touch upon" , "Be next door to" , "Lean against the odd tuba" , "Tuba (anag)" .)