Captivate in French song (7)
I believe the answer is:
enchant
'captivate' is the definition.
('enchant' can be a synonym of 'captivate')
'in french song' is the wordplay.
'in french' becomes 'en' ('en' can be French for 'in' e.g. en France).
'song' becomes 'chant' (I've seen this before**).
'en'+'chant'='ENCHANT'
(Other definitions for enchant that I've seen before include "Delight, cast spell over" , "transport" , "Beguile" , "Bewitch, charm" , "Captivate, charm" .)