Make sure of showing sectarian trouble (9)
I believe the answer is:
ascertain
'make sure of' is the definition.
(thesaurus)
'sectarian trouble' is the wordplay.
'trouble' is an anagram indicator.
'sectarian' is an anagram of 'ASCERTAIN'.
'showing' acts as a link.
(Other definitions for ascertain that I've seen before include "Become aware" , "Find out for sure, determine" , "Verify" , "Establish (the facts)" , "Establish after an investigation or study" .)