Where the Queen entertains most are to run round (9)
I believe the answer is:
stateroom
'where the queen entertains' is the definition.
(room for state events)
'most are to run round' is the wordplay.
'run round' is an anagram indicator (I've seen 'running round' mean this).
'most'+'are'+'to'='mostareto'
'mostareto' anagrammed gives 'STATEROOM'.
(Other definitions for stateroom that I've seen before include "Private cabin on board ship" , "part of palace" , "Luxury cabin on a liner" , "Place fit for a 27 [PRINCE]" , "Large private cabin on a ship" .)