Good man in denial, being horrid (5)
I believe the answer is:
nasty
'horrid' is the definition.
(thesaurus)
'good man in denial' is the wordplay.
'good man' becomes 'st'.
'in' is an insertion indicator.
'denial' becomes 'nay' (nay is a kind of denial).
'st' inserted into 'nay' is 'NASTY'.
'being' acts as a link.
(Other definitions for nasty that I've seen before include "Beastly" , "Horrible" , "Evil" , "Repulsive" , "Horrid, repugnant" .)