Big fireplace, by the sound of it (5)
I believe the answer is:
great
'big' is the definition.
(I know that big can be written as great)
'fireplace by the sound of it' is the wordplay.
'fireplace' becomes 'grate' (grate can mean a fireplace).
'by the sound of it' shows a homophone (sound like).
'grate' sounds like 'GREAT'.
(Other definitions for great that I've seen before include "Extensive; excellent" , "Large, important" , "Huge or outstanding" , "The Plough" , "Very good, excellent" .)