Terribly fat at the back (3)
I believe the answer is:
aft
'at the back' is the definition.
'terribly fat' is the wordplay.
'terribly' is an anagram indicator.
'fat' with letters rearranged gives 'AFT'.
(Other definitions for aft that I've seen before include "Toward's a ship's stern" , "Towards rear (of ship)" , "Sternward" , "back to sailors" , "At the back at sea" .)