Disapprove of old unorthodox popes (6)
I believe the answer is:
oppose
'disapprove of' is the definition.
(thesaurus)
'old unorthodox popes' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'unorthodox' is an anagram indicator.
'popes' anagrammed gives 'ppose'.
'o'+'ppose'='OPPOSE'
(Other definitions for oppose that I've seen before include "Refute - confront" , "Act against" , "Compete with" , "Argue against" , "Resist, go against" .)