Head on board, US sailor coming over (3)
I believe the answer is:
bog
'head on board us' is the definition.
The answer and definition can be both natural objects as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'sailor coming over' is the wordplay.
'sailor' becomes 'gob' (gob is a kind of sailor).
'coming over' says the letters should be written in reverse.
'gob' in reverse letter order is 'BOG'.
Can you help me to learn more?
(Other definitions for bog that I've seen before include "Mire" , "Marsh" , "Wet spongy ground" , "Quagmire" , "Soft muddy area" .)