Relaxed youth should keep one stern (4,4)
I believe the answer is:
laid back
'relaxed' is the definition.
(I've seen this before)
'youth should keep one stern' is the wordplay.
'youth' becomes 'lad' (lad means a young man).
'should keep' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'stern' becomes 'back' (stern is the back of a boat).
'lad' going around 'i' is 'laid'.
'laid'+'back'='LAID-BACK'
(Other definitions for laid back that I've seen before include "not affected by pressure?" , "Relaxed" , "Disinclined to make effort" .)