Not frosty? A fine story! (7)
I believe the answer is:
affable
'not frosty?' is the definition.
(I've seen this in another clue)
'a fine story' is the wordplay.
'fine' becomes 'f' (abbreviation in pencil classifications).
'story' becomes 'fable' (fable is a kind of story).
'a'+'f'+'fable'='AFFABLE'
(Other definitions for affable that I've seen before include "Congenial, easy to walk" , "Genial and cordial" , "Showing friendly warmth" , "Amiable, genial" , "Kindly" .)