Radical old term used first (9)
I believe the answer is:
extremist
'radical' is the definition.
(I know that radical can be written as extremist)
'old term used first' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'used' indicates anagramming the letters.
'first' becomes 'ist' (ist resembles 1st).
'term' anagrammed gives 'trem'.
'ex'+'trem'+'ist'='EXTREMIST'
(Other definitions for extremist that I've seen before include "One far from moderate" , "outwardly fanatical" , "His views are anything but neutral" , "One with very radical views" , "immoderate person" .)