A measure that's backward (3)
I believe the answer is:
aft
'backward' is the definition.
(thesaurus)
'a measure' is the wordplay.
'measure' becomes 'ft' (ft is a kind of measure).
'a'+'ft'='AFT'
'that's' is the link.
(Other definitions for aft that I've seen before include "Near the rear (of ship)" , "near the tail" , "Toward's a ship's stern" , "towards the end" , "At the back of a ship" .)