Old king at home? Smile (4)
I believe the answer is:
grin
'smile' is the definition.
(I know that grin is a more specific form of the action smile)
'old king at home?' is the wordplay.
'old king' becomes 'gr' (Georgius Rex, Latin for King George).
'at home?' becomes 'in' ('I'm in' can mean 'I'm at home').
'gr'+'in'='GRIN'
(Other definitions for grin that I've seen before include "Accept with good grace - . . . . and bear it" , "Look happy" , "11 smile" , "Facial expression showing amusement or pleasure" , "Beam" .)