Ague (5)
I believe the answer is:
fever
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">ague</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for fever that I've seen before include "eg Malaria" , "Heat of disease or extreme excitement" , "A rise in the temperature of the body" , "State of heightened excitement" , "Reason for temperature" .)