Left in order to find stronghold (6)
I believe the answer is:
castle
'stronghold' is the definition.
(I know that castle is a type of citadel)
'left in order' is the wordplay.
I cannot quite see how this works, but
'left' could be 'l' (common abbreviation) and 'l' is found within the answer.
'order' could be 'caste' (I've seen this in other clues) and 'caste' is found in the remaining letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'to find' is the link.
(Other definitions for castle that I've seen before include "Man on board" , "Fortress; rook" , "Move king to safety - stronghold" , "Left's Social Services secretary" , "Lincoln, Ludlow or Leeds" .)