Stop sleeping — we must entertain a king (4)
I believe the answer is:
wake
'stop sleeping' is the definition.
(to wake up is to stop sleeping)
'we must entertain a king' is the wordplay.
'must entertain' means one lot of letters goes inside another.
'king' becomes 'k' (chess abbreviation).
'a'+'k'='ak'
'we' enclosing 'ak' is 'WAKE'.
(Other definitions for wake that I've seen before include "Finnegans 19, Joyce" , "Traditional watch over corpse before burial" , "Boat's trail in water" , "Trail behind a ship" , "Vigil held over a corpse" .)