Period of unusually hot weather (4,4)
I believe the answer is:
heat wave
'period of unusually hot weather' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for heat wave that I've seen before include "Period of unusually warm weather" , "Period of unusually hot weather" , "Continuous spell of hot weather" , "Long spell of hot weather" , "Prolonged period of very warm weather" .)