Plaster mostly held firm (6)
I believe the answer is:
stucco
'plaster' is the definition.
(stucco is a kind of plaster)
'mostly held firm' is the wordplay.
'mostly' means to remove the last letter.
'held' becomes 'stuck'.
'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 "Plaster for decorative work" , "Plaster for coating exterior walls" , "Fine plaster used in cornices, mouldings" , "wall-covering" , "Plaster used for decorative moulding" .)