Cuts back firm plasterwork (6)
I believe the answer is:
stucco
'plasterwork' is the definition.
(I have seen 'Decorative plasterwork ' mean 'stucco' so perhaps 'plasterwork' could also mean 'stucco')
'cuts back firm' is the wordplay.
'back' is a reversal indicator.
'firm' becomes 'co' (a firm is a company).
'cuts' reversed gives 'stuc'.
'stuc'+'co'='STUCCO'
(Other definitions for stucco that I've seen before include "Fine plaster used on wall surfaces" , "Kind of plaster moulded into decorations" , "Durable finish for walls" , "Fine plaster for ornamenting walls etc" , "Plaster for coating exterior walls" .)