At sea most of cargo enters astern (6)
I believe the answer is:
afloat
'at sea' is the definition.
('afloat' can be similar in meaning to 'at sea')
'most of cargo enters astern' is the wordplay.
'most of' means to remove the last letter.
'cargo' becomes 'load' ('load' can be a synonym of 'cargo').
'enters' indicates putting letters inside.
'astern' becomes 'aft' (I've seen this before).
'load' with its last letter removed is 'loa'.
'loa' placed inside 'aft' is 'AFLOAT'.
(Other definitions for afloat that I've seen before include "Bobbing along?" , "With one's head above water" , "Buoyant: solvent" , "Resting on water" , "On board a ship or boat" .)