Lust of French ancestor (6)
I believe the answer is:
desire
'lust' is the definition.
(I've seen this in another clue)
'of french ancestor' is the wordplay.
'of french' becomes 'de' ('of' in French).
'ancestor' becomes 'sire' (sire is a kind of ancestor).
'de'+'sire'='DESIRE'
(Other definitions for desire that I've seen before include "request" , "Long for (something)" , "Reside (anag.)" , "Want, wish for" , "Concupiscence" .)