Overzealous, fanatic (5)
I believe the answer is:
rabid
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">overzealous</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >'<span class="explanation-format__original">fanatic</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(Other definitions for rabid that I've seen before include "Crazy" , "Ferocious" , "Fanatical, infected" , "Zealous" , "Fanatical - hydrophobic" .)