Unusually fat behind (3)
I believe the answer is:
aft
'behind' is the definition.
(aft means towards or at the end of a ship)
'unusually fat' is the wordplay.
'unusually' indicates anagramming the letters.
'fat' with letters rearranged gives 'AFT'.
(Other definitions for aft that I've seen before include "Of ship, fat in the rear" , "Toward the stern of a boat" , "behind boat" , "Towards the stern of a boat" , "At the back at sea" .)