Fortress with unusual dialect (7)
I believe the answer is:
citadel
'fortress' is the definition.
(citadel can mean a fortified city)
'unusual dialect' is the wordplay.
'unusual' indicates anagramming the letters.
'dialect' anagrammed gives 'CITADEL'.
'with' acts as a link.
(Other definitions for citadel that I've seen before include "place of refuge" , "Stronghold into which people could go for shelter during a battle" , "Kremlin" , "Fortress -- dialect (anag)" , "Keep - castle" .)