King referring to case of mogul in citadel (7)
I believe the answer is:
kremlin
'citadel' is the definition.
(I know that kremlin is a type of citadel)
'king referring to case of mogul in' is the wordplay.
'king' becomes 'K' (abbreviation for king in chess).
'referring to' becomes 're' ('re' means regarding or about).
'case of' says to hollow out the word (remove centre letters) (outsides of).
'mogul' with its centre taken out is 'ml'.
'k'+'re'+'ml'+'in'='KREMLIN'
(Other definitions for kremlin that I've seen before include "Moscow citadel" , "Seat of Russia's government" , "Moscow's seat of government" , "Castle" , "Moscow landmark" .)