Cuts up firm plaster (6)
I believe the answer is:
stucco
'plaster' is the definition.
(I know that stucco is a type of plaster)
'cuts up firm' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'firm' becomes 'co' (a firm is a company).
'cuts' back-to-front is 'stuc'.
'stuc'+'co'='STUCCO'
(Other definitions for stucco that I've seen before include "Plaster for coating or moulding" , "wall-covering" , "Plaster for decorative work" , "Fine plaster for decoration" , "Fine plaster for ornamenting walls etc" .)