Citadels soon tumbling down (12)
I believe the answer is:
disconsolate
'down' is the definition.
(I've seen this before)
'citadels soon tumbling' is the wordplay.
'tumbling' indicates anagramming the letters.
'citadels'+'soon'='citadelssoon'
'citadelssoon' anagrammed gives 'DISCONSOLATE'.
(Other definitions for disconsolate that I've seen before include "Sad; beyond comfort" , "Melancholy" , "Deeply sad" , "Stuck with the blues" , "Unhappy and impossible to comfort" .)