Fireplace, almost, is free (6)
I believe the answer is:
gratis
'free' is the definition.
(synonyms)
'fireplace almost is' is the wordplay.
'fireplace' becomes 'grate' (grate can mean a fireplace).
'almost' means to remove the last letter (most of the word but not all of it).
'grate' with its final letter taken away is 'grat'.
'grat'+'is'='GRATIS'
(Other definitions for gratis that I've seen before include "On the house" , "Free (Latin)" , "Costing nothing, free" , "Free, at no cost" , "Free, costs nothing" .)