Bits to fasten up (5)
I believe the answer is:
parts
'bits' is the definition.
(I've seen this before)
'fasten up' is the wordplay.
'fasten' becomes 'strap' (strapping is a kind of fastening).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'strap' reversed gives 'PARTS'.
'to' is the link.
(Other definitions for parts that I've seen before include "Leaves" , "Components of strap" , "bits" , "Strap (anag.)" , "Areas" .)