Winter bug
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">winter bug</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for flu that I've seen before include "Common disease" , "Minor ailment" , "Viral infection (colloq.)" , "In brief, winter illness" , "Influenza, in short" .)