Unmarried son’s left old flames at home (5)
I believe the answer is:
ingle
'old flames at home' is the definition.
(historical term for a fireplace)
'unmarried son's left' is the wordplay.
'unmarried' becomes 'single-' (similar in meaning).
'left' is a deletion indicator (some letters have left).
'single' with 's' taken out is 'INGLE'.
(Other definitions for ingle that I've seen before include "Corner, nook" , "Hearth" , "Catamite" , "Kind of fireplace - get leg in" , "Fireplace for Nigel" .)