A foot behind (3)
I believe the answer is:
aft
'behind' is the definition.
(aft means towards or at the end of a ship)
'a foot' is the wordplay.
'foot' becomes 'ft' (abbreviation).
'a'+'ft'='AFT'
(Other definitions for aft that I've seen before include "near the tail" , "At the back of a ship" , "To the rear on board" , "behind boat" , "behind at sea" .)