Most frosty (6)
I believe the answer is:
iciest
'most frosty' is the definition.
('icy' can be a synonym of 'frosty')
This is all the clue.
(Other definitions for iciest that I've seen before include "Most slippery" , "with minimal friendliness" , "Most cold and frozen" , "Most cold and frosty" , "Most reserved" .)