Keep unusual dialect (7)
I believe the answer is:
citadel
'keep' is the definition.
(thesaurus)
'unusual dialect' is the wordplay.
'unusual' indicates anagramming the letters.
'dialect' is an anagram of 'CITADEL'.
(Other definitions for citadel that I've seen before include "Bastion" , "Urban fortress" , "Fort, fastness" , "Structure in place to protect" , "Kremlin" .)