A French affaire d'honneur failing to reach conclusion is not comme il faut (5)
I believe the answer is:
undue
'not comme il faut' is the definition.
('comme il faut' means necessary or as is expected)
'a french affaire d'honneur failing to reach conclusion' is the wordplay.
'a french' becomes 'un' ('a' in French).
'affaire d'honneur' becomes 'duel' (I've seen this before).
'failing to reach conclusion' means to remove the last letter.
'duel' with its last letter taken off is 'due'.
'un'+'due'='UNDUE'
'is' acts as a link.
(Other definitions for undue that I've seen before include "More than appropriate" , "Extreme" , "Excessive, disproportionate" , "Excessive - unwarranted" , "Not to be paid" .)