Nobleman expected to entertain king (4)
I believe the answer is:
duke
'nobleman' is the definition.
(duke is a kind of nobleman)
'expected to entertain king' is the wordplay.
'expected' becomes 'due' (similar in meaning).
'to entertain' means one lot of letters goes inside another.
'king' becomes 'k' (abbreviation for king in chess).
'due' enclosing 'k' is 'DUKE'.
(Other definitions for duke that I've seen before include "Nickname of John Wayne" , "Member of the nobility" , "noble" , "High hereditary title in Britain" , "Fist (sl.)" .)