Neighbour set up brass instrument (4)
I believe the answer is:
tuba
'brass instrument' is the definition.
(I know that tuba is a type of brass instrument)
'neighbour set up' is the wordplay.
'neighbour' becomes 'abut' (I've seen this before).
'set up' shows that the letters should be reversed in order (in a down clue, letters go up).
'abut' backwards is 'TUBA'.
(Other definitions for tuba that I've seen before include "A bass instrument" , "Musical instrument -- 5 (rev)" , "Large low-pitched brass musical instrument" , "Producer of note" , "Brass band instrument" .)