Unable to proceed, removing last firm plaster (6)
I believe the answer is:
stucco
'plaster' is the definition.
(stucco is a kind of plaster)
'unable to proceed removing last firm' is the wordplay.
'unable to proceed' becomes 'stuck'.
'removing last' means to remove the last letter.
'firm' becomes 'co' (a firm is a company).
'stuck' with its final letter taken away is 'stuc'.
'stuc'+'co'='STUCCO'
(Other definitions for stucco that I've seen before include "wall-covering" , "Plaster for ornamental moulding" , "Fine plaster for moulding into architectural decorations" , "Fine plaster for ornamenting walls etc" , "material for covering wall" .)