Bottom of a vessel (3)
I believe the answer is:
ass
'bottom' is the definition.
('ass' can be a synonym of 'bottom')
'a vessel' is the wordplay.
'vessel' becomes 'ss'.
'a'+'ss'='ASS'
'of' acts as a link.
(Other definitions for ass that I've seen before include "Stupid person or American bum" , "Small hoofed animal" , "An American backside" , "slow-witted person" , "Quadruped; fool" .)