Zealous, fanatic (5)
I believe the answer is:
rabid
I believe this is a double definition.
'zealous' is the first definition.
(I've seen this in another clue)
'fanatic' is the second definition.
(I know that fanatic can be written as rabid)
(Other definitions for rabid that I've seen before include "Violent; hydrophobic" , "Affected by 24 across" , "Crazy" , "Mad like deranged dog" , "Ill from a mad dog bite?" .)