Be too hot (7)
I believe the answer is:
swelter
'be too hot' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for swelter that I've seen before include "Overheat" , "Suffer oppressive heat" , "Suffer when too hot" , "Endure oppressive heat" , "Be too hot; Gormenghast chef" .)