Firm provided partial cover (4)
I believe the answer is:
coif
'partial cover' is the definition.
(type of cap)
'firm provided' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'provided' becomes 'if' (eg 'provided it's sunny' means 'if it's sunny').
'co'+'if'='COIF'
(Other definitions for coif that I've seen before include "what can be made of lawn?" , "Headgear" , "Nun's cap" , "Cap worn by fundamentalist Mormon women: hairstyle" , "Dress the hair" .)