Double up in ruined citadel (9)
I believe the answer is:
duplicate
'double' is the definition.
(I know that double can be written as duplicate)
'up in ruined citadel' is the wordplay.
'in' means one lot of letters goes inside another.
'ruined' indicates anagramming the letters.
'citadel' anagrammed gives 'dlicate'.
'up' put within 'dlicate' is 'DUPLICATE'.
(Other definitions for duplicate that I've seen before include "Double" , "match" , "Increase twofold" , "Clone" , "repeat" .)