Clairvoyant ringing French duke, a Casanova (7)
I believe the answer is:
seducer
'a casanova' is the definition.
(I've seen this before)
'clairvoyant ringing french duke' is the wordplay.
'clairvoyant' becomes 'seer' (I've seen this in another clue).
'ringing' indicates putting letters inside.
'french duke' becomes 'duc' ('duke' in French).
'seer' enclosing 'duc' is 'SEDUCER'.
(Other definitions for seducer that I've seen before include "Wolf" , "One leading sexually astray" , "Don Juan" , "One who takes advantage of the opposite sex" , "Rake" .)