Nice person demonstrated a little energy (9)
I believe the answer is:
provencal
'nice person' is the definition.
(someone from Provence, eg someone from Nice)
'demonstrated a little energy' is the wordplay.
'demonstrated' becomes 'proven' (prove can mean to demonstrate).
'a little energy' becomes 'cal' (short for calorie).
'proven'+'cal'='PROVENCAL'
(Other definitions for provencal that I've seen before include "of southern France" , "Of the region in south-eastern France" , "French language" , "of Langue d'oc" , "French citizen" .)