Keep idle cat active (7)
I believe the answer is:
citadel
'keep' is the definition.
(I've seen this before)
'idle cat active' is the wordplay.
'active' indicates an anagram (the letters are active/moving).
'idle'+'cat'='idlecat'
'idlecat' anagrammed gives 'CITADEL'.
(Other definitions for citadel that I've seen before include "Stronghold (of the Salvation Army?)" , "Kremlin" , "Structure in place to protect" , "Fortress -- dialect (anag)" , "Fortress (in or near city)" .)