By the sound of it, was on highway (4)
I believe the answer is:
road
'highway' is the definition.
(highway is a type of road)
'by the sound of it was on' is the wordplay.
'by the sound of it' shows a homophone (sound like).
'was on' becomes 'rode' (I've seen this in another clue).
'rode' sounds like 'ROAD'.
(Other definitions for road that I've seen before include "Way from A to B" , "motorway?" , "Way to achieving a particular outcome" , "Street" , "For place to anchor" .)