Zealot is in fact a criminal (7)
I believe the answer is:
fanatic
'zealot' is the definition.
(I've seen this before)
'in fact a criminal' is the wordplay.
'criminal' indicates anagramming the letters.
'in'+'fact'+'a'='infacta'
'infacta' with letters rearranged gives 'FANATIC'.
'is' acts as a link.
(Other definitions for fanatic that I've seen before include "Devotee beyond reason" , "Obsessive enthusiast" , "One motivated by irrational enthusiasm" , "Extreme enthusiast" , "enthusiastic" .)