Keep changing dialect (7)
I believe the answer is:
'keep' is the definition.
(I've seen this before)
'changing dialect' is the wordplay.
'changing' is an anagram indicator.
'dialect' is an anagram of 'CITADEL'.
(Other definitions for citadel that I've seen before include "Fortress (in or near city)" , "Keep - castle" , "Fortress overlooking a city" , "Fort, fastness" , "Kremlin" .)