More frosty (5)
I believe the answer is:
icier
'more frosty' is the definition.
('icy' can be a synonym of 'frosty')
This is all the clue.
(Other definitions for icier that I've seen before include "With more hostility" , "Colder even than 31 Across" , "More slippery" , "increasingly frigid" , "with more reserve" .)