Unmarried person abandoning spot of sun in warm spot (5)
I believe the answer is:
ingle
'unmarried person' is the definition.
The definition suggests a singular noun which matches the answer.
'abandoning spot of sun in warm spot' is the wordplay.
I cannot really understand how this works, but
'in' is found in the answer.
This explanation may well be incorrect...
(Other definitions for ingle that I've seen before include "Fire - fireplace" , "Fireplace blaze (Scot.)" , "Fireplace and Irish Times writer" , "Hearth" , "Corner, nook" .)