Frozen stiff after first of frosts (6)
I believe the answer is:
frigid
'frozen' is the definition.
(I've seen this before)
'stiff after first of frosts' is the wordplay.
'stiff' becomes 'rigid' (synonyms).
'after' is a charade indicator (letters next to each other).
'first of frosts' becomes 'f' (1st letter of 'frosts').
'rigid' put after 'f' is 'FRIGID'.
(Other definitions for frigid that I've seen before include "Frosty" , "Very cold; formal in behaviour" , "Cold and unresponsive" , "Very cold in manner" , "Icy" .)