It may keep one warm, although one’s not cool (6)
I believe the answer is:
anorak
'it may keep one warm although one's' is the definition.
The definition suggests a singular noun which matches the answer.
'not cool' is the wordplay.
I cannot quite see how this works, but
'not' could be 'no' (negative) and 'no' is found in the answer.
The remaining letters 'arak' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for anorak that I've seen before include "Nerdy type" , "Socially inept person" , "European currency" , "Arctic jacket: nerd" , "Jacket for outdoors" .)