Old term about first fanatic (9)
I believe the answer is:
extremist
'fanatic' is the definition.
(I've seen this before)
'old term about first' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'about' indicates an anagram.
'first' becomes 'ist' (ist resembles 1st).
'term' with letters rearranged gives 'trem'.
'ex'+'trem'+'ist'='EXTREMIST'
(Other definitions for extremist that I've seen before include "One with views far from the mainstream" , "Revolutionary" , "One with highly non-standard views" , "Ultra" , "His views are anything but neutral" .)