Fanatical, zealous (5)
I believe the answer is:
rabid
This clue is a double definition.
'fanatical' is the first definition.
(I know that fanatical can be written as rabid)
'zealous' is the second definition.
(I've seen this before)
(Other definitions for rabid that I've seen before include "Infected with hydrophobia" , "Affected by hydrophobia" , "Raging (having been bitten by a dog?)" , "Burning" , "Fanatical - hydrophobic" .)