Good-looker in a party turning evil (6)
I believe the answer is:
adonis
'good-looker' is the definition.
(famously attractive youth in Greek mythology)
'a party turning evil' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'turning' says the letters should be written in reverse.
'evil' becomes 'sin' (synonyms).
'sin' in reverse letter order is 'nis'.
'a'+'do'+'nis'='ADONIS'
'in' is the link.
(Other definitions for adonis that I've seen before include "striking youth" , "legendary Greek noted for good looks" , "one deemed to be beautiful" , "Handsome Gk. youth" , "lovely chap" .)