Breaks up the band (5)
I believe the answer is:
parts
'breaks' is the definition.
(part can mean to split or break up)
'up the band' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'the band' becomes 'strap' (both can mean a strip of material).
'strap' in reverse letter order is 'PARTS'.
(Other definitions for parts that I've seen before include "bits" , "Strap (anag.)" , "Areas" , "Leaves" , "Components of strap" .)