Relative eating one seed (5)
'seed' is the definition.
(grain is a kind of seed)
'relative eating one' is the wordplay.
'relative' becomes 'gran' (I've seen this in other clues**).
'eating' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'gran' placed around 'i' is 'GRAIN'.
(Other definitions for grain that I've seen before include "Enter into combat with", "Cereal seed", "eg Barley", "Pattern of lines in wood", "Small quantity (of corn etc)".)