Queen's Gallery exhibits a Constable retrospective, entertaining stars (9)
I believe the answer is:
cleopatra
'queen's' is the definition.
(Cleopatra was an Egyptian queen)
'gallery exhibits a constable retrospective entertaining stars' is the wordplay.
'gallery exhibits' becomes 'art' (I've seen this in another clue).
'constable' becomes 'PC' (police constable).
'retrospective' says the letters should be written in reverse.
'entertaining' is an insertion indicator (inviting in, as for dinner).
'stars' becomes 'Leo' (constellation).
'art'+'a'+'pc'='artapc'
'artapc' in reverse letter order is 'cpatra'.
'cpatra' enclosing 'leo' is 'CLEOPATRA'.
(Other definitions for cleopatra that I've seen before include "Film" , "Old queen of Egypt" , "drama queen" , "Queen of Egypt, loved by Caesar and Mark Antony" , "Her Needle is on the Embankment in London" .)