The sections strap up (5)
I believe the answer is:
'the sections' is the definition.
('part' can be a synonym of 'section')
'strap up' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'strap' reversed gives 'PARTS'.
(Other definitions for parts that I've seen before include "Areas" , "Components of strap" , "Leaves" , "Strap (anag.)" , "bits" .)