Wonderful fireplace, say (5)
I believe the answer is:
great
'wonderful' is the definition.
(I know that wonderful can be written as great)
'fireplace say' is the wordplay.
'fireplace' becomes 'grate' (grate can mean a fireplace).
'say' shows a homophone (sound like).
'grate' sounds like 'GREAT'.
(Other definitions for great that I've seen before include "Of major significance - very good" , "Large; fine" , "Mum's dad's mum, for example" , "Huge or outstanding" , "Very good, excellent" .)