Fanatical detective to stop revolutionary (5)
I believe the answer is:
rabid
'fanatical' is the definition.
(I know that fanatical can be written as rabid)
'detective to stop revolutionary' is the wordplay.
'detective' becomes 'di' (Detective Inspector abbreviation).
'to stop' becomes 'bar' (bar can mean to stop or prevent).
'revolutionary' says the letters should be written backwards.
'di'+'bar'='dibar'
'dibar' reversed gives 'RABID'.
(Other definitions for rabid that I've seen before include "Violent; hydrophobic" , "Ferocious" , "Mad; raging" , "Fanatical, infected" , "Extremely fanatical" .)