Unplanned citadel can get disorganised (10)

I believe the answer is:

accidental

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'unplanned' is the definition.
(similar in meaning)

'citadel can get disorganised' is the wordplay.
'citadel can' anagrams to 'ACCIDENTAL'.
But, I am not sure how the anagram is indicated.

Can you help me to learn more?

(Other definitions for accidental that I've seen before include "without planning" , "Happening by chance" , "indication to modify tone slightly" , "Flat, say" , "Chance, inadvertent" .)

I've seen this clue in The Independent.

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