Period of unusually hot weather (4,4)
I believe the answer is:
heat wave
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">period of unusually hot weather</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)