Sectarian party last to cast stigma off (9)
I believe the answer is:
dogmatist
'sectarian' is the definition.
(thesaurus)
'party last to cast stigma off' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'last to' indicates one should take the final letters.
'off' indicates an anagram.
The last letter of 'cast' is 't'.
't'+'stigma'='tstigma'
'tstigma' is an anagram of 'gmatist'.
'do'+'gmatist'='DOGMATIST'
(Other definitions for dogmatist that I've seen before include "stickler" , "bigot" , "opinionated type" .)