Stitch up back and sides (4)
I believe the answer is:
purl
'stitch' is the definition.
(purl is a kind of stitch)
'up back and sides' is the wordplay.
'back' shows that the letters should be reversed in order.
'and' says to put letters next to each other.
'sides' becomes 'rl' (I've seen this in other clues).
'up' back-to-front is 'pu'.
'pu'+'rl'='PURL'
(Other definitions for purl that I've seen before include "Knitting stitch type" , "Twisted gold wire" , "form loop" .)