Son will keep one set (4)
I believe the answer is:
laid
'set' is the definition.
('laid' can be a synonym of 'set')
'son will keep one' is the wordplay.
'son' becomes 'lad' (I've seen this before).
'will keep' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'lad' going around 'i' is 'LAID'.
(Other definitions for laid that I've seen before include "Produced eggs" , "Deposited" , "settled" , "(Of dining table) set" , "Placed horizontally on a surface" .)