Negative word about good person is unpleasant (5)
I believe the answer is:
nasty
'unpleasant' is the definition.
(associated in meaning)
'negative word about good person' is the wordplay.
'negative word' becomes 'nay' (archaic form of 'no').
'about' is an insertion indicator.
'good person' becomes 'st' (saint).
'nay' going around 'st' is 'NASTY'.
'is' is the link.
(Other definitions for nasty that I've seen before include "Disgusting" , "Vile" , "Mean" , "Offensive or malicious" , "Foul" .)