In fact a wild enthusiast (7)
I believe the answer is:
fanatic
'enthusiast' is the definition.
(I know that enthusiast can be written as fanatic)
'in fact a wild' is the wordplay.
'wild' is an anagram indicator.
'in'+'fact'+'a'='infacta'
'infacta' is an anagram of 'FANATIC'.
(Other definitions for fanatic that I've seen before include "crank" , "Narrow-minded enthusiast" , "One motivated by irrational enthusiasm" , "Over-enthusiastic person" , "Overzealous person" .)