Mostly cut back on material for curtains (5)
I believe the answer is:
scrim
'material for curtains' is the definition.
(material used for theatre curtains)
'mostly cut back on' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'cut back on' becomes 'scrimp' (to scrimp on something is to be economical with it).
'scrimp' with its final letter taken off is 'SCRIM'.
(Other definitions for scrim that I've seen before include "Open-weave muslin fabric" , "material" , "A loosely-woven cotton fabric" .)