Old term given new twist by one small in zealotry (9)
I believe the answer is:
extremism
'zealotry' is the definition.
(thesaurus)
'old term given new twist by one small' is the wordplay.
I cannot quite understand how this works, but
'old' could be 'ex' (prefix meaning former or onetime) and 'ex' is found within the answer.
an anagram of 'term' is 'trem' which is present in the answer.
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
'small' could be 'sm' and 'sm' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'in' acts as a link.
Can you help me to learn more?
(Other definitions for extremism that I've seen before include "Ultra-radical views" , "The holding of way-out ideas" , "Immoderate view" , "Radical party's policy" , "The holding of far from moderate views" .)