Touch up some brass (4)
I believe the answer is:
tuba
'some brass' is the definition.
(tuba is a kind of brass)
'touch up' is the wordplay.
'touch' becomes 'abut' (abutting is a kind of touching).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'abut' back-to-front is 'TUBA'.
(Other definitions for tuba that I've seen before include "Big brass instrument" , "item band might play" , "Lowest brass wind instrument" , "horn" , "A bass instrument" .)