A back-cloth? (3)
I believe the answer is:
per
'a' is the definition.
'back-cloth?' is the wordplay.
'back' shows that the letters should be reversed in order.
'cloth?' becomes 'rep' (rep is a kind of cloth).
'rep' reversed gives 'PER'.
(Other definitions for per that I've seen before include "Preposition meaning for each" , "For each or every" , "According to" , "Preposition indicating by means of" .)