A back-cloth? (3)
I believe the answer is:
Here is my best explanation:
'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 "For each or by means of", "Through", "Preposition meaning for each", "By means of", "A", "Preposition indicating by means of", "For each or every" and "According to".)