Old application for waterway (4)
I believe the answer is:
ouse
'for waterway' is the definition.
(the River Ouse)
'old application' is the wordplay.
'old' becomes 'o' (abbreviation).
'application' becomes 'use' (I've seen this before).
'o'+'use'='OUSE'
(Other definitions for ouse that I've seen before include "River at Bedford" , "River flowing through York" , "River of 1 across" , "River that joins the Trent to form the Humber" , "Lewes's river" .)