Large recess in fireplace, so to speak (5)
I believe the answer is:
Here is my best explanation:
'large' is the definition.
(I've seen this before)
'recess in fireplace so to speak' is the wordplay.
'recess in fireplace' becomes 'grate'.
'so to speak' indicates a 'sounds like' (homophone) clue.
'grate' sounds like 'GREAT'.
(Other definitions for great that I've seen before include "The Plough", "Huge or outstanding", "Very good or very big", "and 12 Down: Powerful dog", "Large; fine", "Big, large", "Weighty", "Marvellous; big", "Important; immense" and "Very good, excellent".)