Some French duke is failure (3)
I believe the answer is:
dud
'failure' is the definition.
(dud is a kind of failure)
'some french duke' is the wordplay.
'some french' becomes 'du'.
'duke' becomes 'd'.
'du'+'d'='DUD'
'is' is the link.
(Other definitions for dud that I've seen before include "Failing to work or meet standard (colloq.)" , "Failure; washout" , "One failing to work, explode" , "A palindromic failure" , "Thing that fails to work" .)