The same citadel in trouble (9)
I believe the answer is:
identical
'the same' is the definition.
(I've seen this in another clue)
'citadel in trouble' is the wordplay.
'trouble' indicates anagramming the letters.
'citadel'+'in'='citadelin'
'citadelin' with letters rearranged gives 'IDENTICAL'.
(Other definitions for identical that I've seen before include "Exactly alike, the same in every detail" , "Similar" , "Like two peas in a pod" , "uniform" , "Matching" .)