Stern and terribly fat (3)
I believe the answer is:
aft
'stern' is the definition.
(similar in meaning)
'terribly fat' is the wordplay.
'terribly' is an anagram indicator.
'fat' with letters rearranged gives 'AFT'.
'and' acts as a link.
(Other definitions for aft that I've seen before include "At the back of a ship" , "Towards the stern of a boat" , "Behind (naval)" , "Toward's a ship's stern" , "At the back at sea" .)