Heat units, ignoring hot conditions (5)
I believe the answer is:
terms
'conditions' is the definition.
('term' can be a synonym of 'condition')
'heat units ignoring hot' is the wordplay.
'heat units' becomes 'therms' (therm is a kind of heat unit).
'ignoring' suggests deleting specific letters.
'hot' becomes 'h'.
'therms' with 'h' taken out is 'TERMS'.
(Other definitions for terms that I've seen before include "Stipulated requirements" , "Opportunities to study" , "Conditions; fee" , "Times" , "Phrases" .)