Overzealous (5)
I believe the answer is:
rabid
'overzealous' is the definition.
(I know that overzealous can be written as rabid)
This is the entire clue.
(Other definitions for rabid that I've seen before include "Violent; hydrophobic" , "Burning" , "Ill from a mad dog bite?" , "Extreme, fanatical" , "Extremely fanatical" .)