The bits to strap up (5)
I believe the answer is:
parts
'the bits' is the definition.
(I've seen this before)
'strap up' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'strap' in reverse letter order is 'PARTS'.
'to' acts as a link.
(Other definitions for parts that I've seen before include "Components of strap" , "Leaves" , "bits" , "Areas" , "Strap (anag.)" .)