Old king gives primate some food (5)
I believe the answer is:
grape
'some food' is the definition.
(grape is a kind of food)
'old king gives primate' is the wordplay.
I cannot quite understand how this works, but
'king' could be 'gr' (short for Georgius Rex, King George in Latin) and 'gr' is found within the answer.
'primate' could be 'ape' (large primate) and 'ape' is found within the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for grape that I've seen before include "Wine fruit" , "Vineyard fruit" , "Pager (anag.)" , "Fruit from bunch" , "Green/black fruit" .)