Abnormally high temperature (5)
I believe the answer is:
fever
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">abnormally high temperature</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 fever that I've seen before include "Early part of flu" , "Pyrexia" , "State of heightened excitement" , "Reason for temperature" , "(Illness causing) high temperature" .)