Overzealous, fanatic (5)
I believe the answer is:
rabid
I believe this clue is a double definition.
'overzealous' is the first definition.
(synonyms)
'fanatic' is the second definition.
(synonyms)
(Other definitions for rabid that I've seen before include "Crazy" , "Ferocious" , "Fanatical, infected" , "Zealous" , "Fanatical - hydrophobic" .)