Nice man Peter, out to show overindulgence (12)
I believe the answer is:
intemperance
'overindulgence' is the definition.
(thesaurus)
'nice man peter out' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'nice'+'man'+'peter'='nicemanpeter'
'nicemanpeter' with letters rearranged gives 'INTEMPERANCE'.
'to show' is the link.
(Other definitions for intemperance that I've seen before include "lack of restraint" , "Unrestrained alcohol consumption" , "bingeing" , "excess drunkenness?" .)