Stronghold and loch in order (6)
I believe the answer is:
castle
'stronghold' is the definition.
(I know that castle is a type of citadel)
'loch in order' is the wordplay.
'loch' becomes 'l'.
'in' indicates putting letters inside.
'order' becomes 'caste' (type of social rank or order).
'l' placed inside 'caste' is 'CASTLE'.
'and' acts as a link.
(Other definitions for castle that I've seen before include "eg Edinburgh or Windsor" , "Move rook and king in the same turn" , "old building?" , "ancient building?" , "Chess piece and grand house" .)