Holy person in denial is unpleasant (5)
I believe the answer is:
nasty
'unpleasant' is the definition.
(nasty can mean unpleasant or horrible)
'holy person in denial' is the wordplay.
'holy person' becomes 'st' (short for saint).
'in' is an insertion indicator.
'denial' becomes 'nay' (nay is a kind of denial).
'st' going into 'nay' is 'NASTY'.
'is' acts as a link.
(Other definitions for nasty that I've seen before include "One who sees the future" , "Offensive or malicious" , "Highly unpleasant" , "Disgusting" , "grave" .)