Terribly fat behind (3)
I believe the answer is:
aft
'behind' is the definition.
(aft means towards or at the end of a ship)
'terribly fat' is the wordplay.
'terribly' indicates anagramming the letters.
'fat' with letters rearranged gives 'AFT'.
(Other definitions for aft that I've seen before include "At the back at sea" , "Towards the stern of a boat" , "At the back of a ship" , "Farther back on board" , "near the tail" .)