Get to know sectarian in an unusual way (9)
I believe the answer is:
'get to know' is the definition.
(ascertaining is a kind of getting to know)
'sectarian in an unusual way' is the wordplay.
'in an unusual way' indicates anagramming the letters.
'sectarian' anagrammed gives 'ASCERTAIN'.
(Other definitions for ascertain that I've seen before include "Become aware" , "Establish, find out" , "Establish (the facts)" , "Learn" , "Find out for sure, determine" .)