Smart criminal hosting party in realm of emperor (7)
I believe the answer is:
tsardom
'realm of emperor' is the definition.
'smart criminal hosting party' is the wordplay.
'criminal' indicates anagramming the letters.
'hosting' means one lot of letters goes inside another.
'party' becomes 'do' (do is a kind of party).
'smart' with letters rearranged gives 'tsarm'.
'tsarm' enclosing 'do' is 'TSARDOM'.
'in' acts as a link.
(Other definitions for tsardom that I've seen before include "Russian ruler's rank or status" , "what Ivan had?" , "Russian emperor's rule" , "Russian emperor's state" , "old ruler's office" .)