Come up against reversed instrument (4)
I believe the answer is:
tuba
'instrument' is the definition.
(I know that tuba is a type of instrument)
'come up against reversed' is the wordplay.
'come up against' becomes 'abut' (I've seen this in another clue).
'reversed' says the letters should be written backwards.
'abut' reversed gives 'TUBA'.
(Other definitions for tuba that I've seen before include "Deep brass instrument" , "Big brass instrument" , "Stop" , "member of band" , "item band might play" .)