A back-cloth? (3)
I believe the answer is:
Here is my best explanation:
'a' is the definition.
'back-cloth?' is the subsidiary indication.
'back' is a reversal indicator.
'cloth?' becomes 'rep' (rep is a kind of cloth).
'rep' written backwards gives 'per'.
(Other definitions for per that I've seen before include "A", "By means of", "Through", "For each or by means of", "Preposition indicating by means of", "For each or every" and "Preposition meaning for each".)