Marker at sea young lad picked up (4)
I believe the answer is:
buoy
'marker at sea' is the definition.
(floating marker in the sea)
'young lad picked up' is the wordplay.
'young lad' becomes 'boy' (lad can mean informally mean a boy).
'picked up' shows a homophone (sound like).
'boy' is a homophone of 'BUOY'.
(Other definitions for buoy that I've seen before include "Marine float" , "dolphin" , "Floating guide marker" , "Anchored floater" , "Floating navigation mark" .)