Idle cat moving around fortress (7)
I believe the answer is:
citadel
'fortress' is the definition.
(citadel can mean a fortified city)
'idle cat moving' is the wordplay.
'moving' indicates anagramming the letters (letters are moved around).
'idle'+'cat'='idlecat'
'idlecat' anagrammed gives 'CITADEL'.
'around' is the link.
(Other definitions for citadel that I've seen before include "Highly-secure building" , "Urban stronghold - dialect (anag)" , "secure place" , "Fortress (in or near city)" , "Bastion" .)