A foot back (3)
I believe the answer is:
aft
'back' is the definition.
(I've seen this before)
'a foot' is the wordplay.
'foot' becomes 'ft' ('ft' can be a synonym of 'foot').
'a'+'ft'='AFT'
(Other definitions for aft that I've seen before include "Toward's a ship's stern" , "Toward the stern of a boat" , "behind at sea" , "towards the end" , "Towards the back on board" .)