Old term - it's upsetting fanatic (9)
I believe the answer is:
extremist
'fanatic' is the definition.
(I know that fanatic can be written as extremist)
'old term it's upsetting' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'upsetting' indicates anagramming the letters.
'term'+'its'='termits'
'termits' with letters rearranged gives 'tremist'.
'ex'+'tremist'='EXTREMIST'
(Other definitions for extremist that I've seen before include "Views far beyond the norm" , "Ultra" , "outwardly fanatical" , "One with very radical views" , "One with views very far from consensus" .)