Disapprove of old unpredictable popes (6)
I believe the answer is:
oppose
'disapprove of' is the definition.
(thesaurus)
'old unpredictable popes' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'unpredictable' is an anagram indicator.
'popes' is an anagram of 'ppose'.
'o'+'ppose'='OPPOSE'
(Other definitions for oppose that I've seen before include "match" , "Take stand against, resist" , "Resist, be hostile to" , "Resist; disapprove of" , "Resist; play against" .)