Band splits up (5)
I believe the answer is:
strap
'band' is the definition.
(strap is a kind of band)
'splits up' is the wordplay.
'splits' becomes 'parts' ('part' can be a synonym of 'split').
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'parts' backwards is 'STRAP'.
(Other definitions for strap that I've seen before include "Tie" , "something to hang on to" , "Parts of leather belt" , "Parts (anag.)" , "e.g. Belt" .)