Almost stuck with firm plaster (6)
I believe the answer is:
stucco
'plaster' is the definition.
(stucco is a kind of plaster)
'almost stuck with firm' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'with' means one lot of letters go next to another.
'firm' becomes 'co' (a firm is a company).
'stuck' with its last letter taken off is 'stuc'.
'stuc'+'co'='STUCCO'
(Other definitions for stucco that I've seen before include "Plaster for coating outside walls" , "Decorative external plaster" , "Fine type of plaster" , "Fine plaster for moulding into architectural decorations" , "Kind of plaster moulded into decorations" .)