Sailor accounting for most of ship's weight (3)
I believe the answer is:
tar
'sailor' is the definition.
(slang term for a sailor)
'most of ship's weight' is the wordplay.
'most of' means to remove the last letter.
'ship's weight' becomes 'tare' (the weight of a ship when empty).
'tare' with its last letter taken off is 'TAR'.
'accounting for' is the link.
(Other definitions for tar that I've seen before include "Thick, black and sticky liquid" , "Cigarette residue - old word for sailor" , "Thick, sticky liquid distilled from coal" , "Black viscid substance" , "Viscous mixture - sailor" .)