Old king gets primate something to eat (5)
I believe the answer is:
grape
'something to eat' is the definition.
(grape is a kind of food)
'old king gets primate' is the wordplay.
'old king' becomes 'GR' (Georgius Rex, Latin for King George).
'gets' means one lot of letters go next to another.
'primate' becomes 'ape' (large primate).
'gr'+'ape'='GRAPE'
(Other definitions for grape that I've seen before include "In Elgin rummage about in the dark" , "Vine berry" , "Small fruit; type of shot" , "Vineyard fruit" , "Fruit from bunch" .)