Nancy's right to support American expert (6)
I believe the answer is:
adroit
'expert' is the definition.
(adroit can mean expert or masterful)
'nancy's right to support american' is the wordplay.
'nancy's right' becomes 'droit' ('right' in French - Nancy is a French city).
'to support' says to put letters next to each other (in a down clue, some letters hold others up).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'droit' after 'a' is 'ADROIT'.
(Other definitions for adroit that I've seen before include "Canny" , "Skilful, expert" , "Clever - crafty" , "Quick and clever in action or thought" , "resourceful" .)