Requests fool to entertain king (4)
I believe the answer is:
asks
'requests' is the definition.
('ask' can be a synonym of 'request')
'fool to entertain king' is the wordplay.
'fool' becomes 'ass' (ass is a kind of fool**).
'to entertain' means one lot of letters goes inside another.
'king' becomes 'k'.
'ass' placed around 'k' is 'ASKS'.
(Other definitions for asks that I've seen before include "Makes an inquiry" , "Requests information about" , "is curious" , "Inquiries" , "Begs" .)