Hence cover for firm (5)
I believe the answer is:
solid
'for firm' is the definition.
(synonyms)
'hence cover' is the wordplay.
'hence' becomes 'so'.
'cover' becomes 'lid' (lid is a kind of cover).
'so'+'lid'='SOLID'
(Other definitions for solid that I've seen before include "Not hollow, the same throughout" , "Hard; three dimensional" , "Firmly constructed" , "Not hollow or liquid" , "continuous" .)