Chosen key to the French court (5)
I believe the answer is:
elect
'chosen' is the definition.
('elect' as an adjective can mean chosen)
'key to the french court' is the wordplay.
'key' becomes 'e' (I've seen this in other clues).
'to the french' becomes 'le'.
'court' becomes 'ct'.
'e'+'le'+'ct'='ELECT'
(Other definitions for elect that I've seen before include "Choose by ballot" , "Choose by vote" , "Preferred" , "Make" , "Put in power by voting" .)