Not very warm? That’s great! (4)
I believe the answer is:
cool
This clue is a double definition.
'not very warm?' is the first definition.
(I've seen this before)
'great' is the second definition.
('cool' can be a synonym of 'great')
'that's' is the link.
(Other definitions for cool that I've seen before include "Casual and confident - not heated" , "Calm; chilly" , "Rain heavily (colloq.)" , "Moderate" , "Chilled" .)