Coat mostly held firm (6)
I believe the answer is:
stucco
'coat' is the definition.
(I know that stucco is a more specific form of coat)
'mostly held firm' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'held' becomes 'stuck'.
'firm' becomes 'co' (a firm is a company).
'stuck' with its last letter removed is 'stuc'.
'stuc'+'co'='STUCCO'
(Other definitions for stucco that I've seen before include "Fine plaster for ornamenting walls etc" , "Coating material" , "Cement or plaster used for coating walls" , "Plaster for coating outside walls" , "material for covering wall" .)