Frozen stiff - below Fahrenheit! (6)
I believe the answer is:
frigid
'frozen' is the definition.
(I know that frozen can be written as frigid)
'stiff below fahrenheit' is the wordplay.
'stiff' becomes 'rigid' (synonyms).
'below' says to put letters next to each other (in a down clue, some letters go below others).
'fahrenheit' becomes 'f' (abbreviation).
'rigid' after 'f' is 'FRIGID'.
(Other definitions for frigid that I've seen before include "Cold and unresponsive" , "Very cold; formal in behaviour" , "Passionless, unresponsive" , "Lacking affection or warmth" , "Frosty" .)