Back-to-front rein splits (5)
I believe the answer is:
parts
'splits' is the definition.
('part' can be a synonym of 'split')
'back-to-front rein' is the wordplay.
'back-to-front' says the letters should be written in reverse.
'rein' becomes 's trap' (I've seen this in another clue).
'strap' back-to-front is 'PARTS'.
(Other definitions for parts that I've seen before include "bits" , "Areas" , "Strap (anag.)" , "Components of strap" , "Leaves" .)