Be up against retro brass instrument (4)
I believe the answer is:
abut
'be up against' is the definition.
(to neighbour something or go beside it)
'retro brass instrument' is the wordplay.
'retro' is a reversal indicator (retro is Latin for 'backwards').
'brass instrument' becomes 'tuba' (I've seen this in another clue).
'tuba' written backwards gives 'ABUT'.
(Other definitions for abut that I've seen before include "Be next to -- share a boundary" , "Adjoin, press up against" , "Join" , "Adjoin -- 19 (rev)" , "Tuba (anag)" .)