Set about one youth (4)
I believe the answer is:
laid
'set' is the definition.
('laid' can be a synonym of 'set')
'about one youth' is the wordplay.
'about' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'youth' becomes 'lad' (I've seen this before).
'i' inserted inside 'lad' is 'LAID'.
(Other definitions for laid that I've seen before include "Put down like dial" , "(Of dining table) set" , "Produced eggs" , "Produced (an egg)" , "Deposited" .)