Agreement in parts of Louisiana (3)
I believe the answer is:
oui
'agreement' is the definition.
('yes' in French)
'in parts of louisiana' is the wordplay.
'in parts of' indicates the answer is hidden within the clue.
'OUI' is hidden within 'louisiana'.
(Other definitions for oui that I've seen before include "Yes, madame" , "Certainly, monsieur (3)" , "Yes, monsieur" , "Yes, mademoiselle" .)