Nautical item is on top of vessel, not bottom (7)
I believe the answer is:
capstan
'vessel not bottom' is the definition.
I can't tell whether this defines the answer.
'nautical item is on top' is the wordplay.
'nautical item' becomes 'stan' (I can't explain this - if you can you should give a lot more credence to this answer).
'is on' means one lot of letters go next to another.
'top' becomes 'cap' (both can mean to beat or outdo).
'stan' put after 'cap' is 'CAPSTAN'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for capstan that I've seen before include "Cylinder for winding ropes/cables" , "Winding mechanism" , "Dockside rope winder" , "Machine for hauling in ropes" , "Apparatus to wind cable" .)