Idle act destroyed fortress (7)
I believe the answer is:
citadel
'fortress' is the definition.
(citadel can mean a fortified city)
'idle act destroyed' is the wordplay.
'destroyed' indicates anagramming the letters.
'idle'+'act'='idleact'
'idleact' with letters rearranged gives 'CITADEL'.
(Other definitions for citadel that I've seen before include "Keep - castle" , "place of refuge" , "Stronghold into which people could go for shelter during a battle" , "Kremlin" , "secure place" .)