Unpleasant refusal to admit a saint (5)
I believe the answer is:
nasty
'unpleasant' is the definition.
(nasty can mean unpleasant or horrible)
'refusal to admit a saint' is the wordplay.
'refusal' becomes 'nay' (archaic word for 'no').
'to admit' is an insertion indicator.
'a saint' becomes 'st' (abbreviation).
'nay' enclosing 'st' is 'NASTY'.
(Other definitions for nasty that I've seen before include "Highly repugnant" , "Repulsive" , "Mean" , "grave" , "Highly unpleasant" .)