Marker at sea young chap picked up (4)
I believe the answer is:
buoy
'marker at sea' is the definition.
(floating marker in the sea)
'young chap picked up' is the wordplay.
'young chap' becomes 'boy' (I've seen this in another clue).
'picked up' shows a homophone (sound like).
'boy' sounds like 'BUOY'.
(Other definitions for buoy that I've seen before include "Warning float" , "Floating guide marker" , "Floating sea marker" , "To raise" , "Warning floater" .)