Bottom's part of a ship (3)
I believe the answer is:
ass
'bottom's' is the definition.
(synonyms)
'a ship' is the wordplay.
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'a'+'ss'='ASS'
'part of' is the link.
(Other definitions for ass that I've seen before include "Stupid fellow" , "'The law is a - - - a idiot' (Mr Bumble)" , "Quadruped; fool" , "Buffoon" , "Turkey" .)