Less warm (6)
I believe the answer is:
cooler
'less warm' is the definition.
('cool' can be the opposite of 'warm' so perhaps 'cooler' is reasonable for 'less warm')
This is all the clue.
(Other definitions for cooler that I've seen before include "Fan" , "It's not so hot" , "less enthusiastic" , "Nick" , "Prison - less warm" .)