Fat animal seen in port (8)
I believe the answer is:
larboard
'port' is the definition.
(archaic nautical term for port)
'fat animal seen in' is the wordplay.
'fat' becomes 'lard' (lard is a kind of fat).
'animal' becomes 'boar' (I've seen this before).
'seen in' means one lot of letters goes inside another.
'lard' enclosing 'boar' is 'LARBOARD'.
(Other definitions for larboard that I've seen before include "Sailor's left" , "Side of ship for Labrador" , "Port side" , "left at sea" , "Port; labrador (anag.)" .)