Ascertain changes, becoming bigoted? (9)
I believe the answer is:
sectarian
'bigoted?' is the definition.
('sectarian' can be similar in meaning to 'bigoted')
'ascertain changes' is the wordplay.
'changes' indicates anagramming the letters.
'ascertain' is an anagram of 'SECTARIAN'.
'becoming' is the link.
(Other definitions for sectarian that I've seen before include "Prejudiced against other religions? So ascertain" , "Ascertain to be with religious prejudice" , "Concerning one group" , "Adhering to a faction or doctrine" , "Of a religious faction" .)