Be adopting strange term for fanatic (9)
I believe the answer is:
extremist
'fanatic' is the definition.
(thesaurus)
'be adopting strange term' is the wordplay.
'be' becomes 'exist' ('exist' can be a synonym of 'be').
'adopting' means one lot of letters goes inside another.
'strange' is an anagram indicator.
'term' anagrammed gives 'trem'.
'exist' placed around 'trem' is 'EXTREMIST'.
'for' is the link.
(Other definitions for extremist that I've seen before include "Far left- or right-winger" , "One with views far from the mainstream" , "immoderate person" , "His views are anything but neutral" , "Revolutionary" .)