Temperature firm at the heart of mildew (4)
I believe the answer is:
cold
'temperature' is the definition.
(cold is a kind of temperature)
'firm at the heart of mildew' is the wordplay.
'firm' becomes 'co' (abbreviation for company).
'at' means one lot of letters go next to another.
'the heart of' indicates the central letters.
The centre of 'mildew' is 'ld'.
'co'+'ld'='COLD'
(Other definitions for cold that I've seen before include "Chilly; minor ailment" , "Unfriendly" , "Unfeeling" , "See 12" , "Chilly; unemotional" .)