King heading off mischief-maker in Russian citadel (7)
I believe the answer is:
kremlin
'russian citadel' is the definition.
(building in Moscow)
'king heading off mischief-maker' is the wordplay.
'king' becomes 'K' (abbreviation for king in chess).
'heading off' means to remove the first letter.
'mischief-maker' becomes 'gremlin' (magical creature).
'gremlin' with its initial letter taken off is 'remlin'.
'k'+'remlin'='KREMLIN'
'in' acts as a link.
(Other definitions for kremlin that I've seen before include "Seat of government in Moscow" , "fortress" , "Castle" , "centre of government" , "Seat of Russian government" .)