Performer one treats badly (7)
I believe the answer is:
artiste
'performer' is the definition.
(artiste is a kind of performer)
'one treats badly' is the wordplay.
'one' becomes 'i' (Roman numeral).
'badly' is an anagram indicator.
'i'+'treats'='itreats'
'itreats' is an anagram of 'ARTISTE'.
(Other definitions for artiste that I've seen before include "'Public performer, dancer or singer (7)'" , "Professional performer" , "Tears it (anag.)" , "Trouper" , "Performers" .)