A foot backward (3)
I believe the answer is:
aft
'backward' is the definition.
(I've seen this in another clue)
'a foot' is the wordplay.
'foot' becomes 'ft' (synonyms).
'a'+'ft'='AFT'
(Other definitions for aft that I've seen before include "towards the end" , "Towards the rear at sea" , "Near the stern" , "Sternward" , "At the back of a ship" .)