Not initially alone in fireplace (5)
I believe the answer is:
ingle
'fireplace' is the definition.
(I've seen this before)
'not initially alone' is the wordplay.
'not initially' means to remove the first letter.
'alone' becomes 'single' (synonyms).
'single' with its first letter removed is 'INGLE'.
'in' acts as a link.
(Other definitions for ingle that I've seen before include "Kind of fireplace - get leg in" , "Fireplace in a room" , "A fireside nook - get leg in" , "Fireplace for Nigel" , "Domestic fireplace" .)