A zealot - in fact a loony (7)
I believe the answer is:
fanatic
'a zealot' is the definition.
(I've seen this before)
'in fact a loony' is the wordplay.
'loony' indicates an anagram.
'in'+'fact'+'a'='infacta'
'infacta' is an anagram of 'FANATIC'.
(Other definitions for fanatic that I've seen before include "Excessively devoted" , "enthusiastic" , "One motivated by irrational enthusiasm" , "Overzealous person" , "Bigot - enthusiast" .)