In fact, a strange enthusiast (7)
I believe the answer is:
fanatic
'enthusiast' is the definition.
(I know that fanatic is a type of enthusiast)
'in fact a strange' is the wordplay.
'strange' indicates anagramming the letters.
'in'+'fact'+'a'='infacta'
'infacta' is an anagram of 'FANATIC'.
(Other definitions for fanatic that I've seen before include "Overzealous person" , "Excessively devoted" , "Narrow-minded enthusiast" , "Bigot - enthusiast" , "Devotee beyond reason" .)