Actors seen next to the French stronghold (6)
I believe the answer is:
castle
'stronghold' is the definition.
(I know that castle is a type of citadel)
'actors seen next to the french' is the wordplay.
'actors' becomes 'cast' (group of actors).
'seen next to' says to put letters next to each other.
'the french' becomes 'le' ('the' in French).
'cast'+'le'='CASTLE'
(Other definitions for castle that I've seen before include "Left's Social Services secretary" , "Chess piece and fortified building" , "Stronghold - chessman" , "Citadel" , "Fort" .)