Conductor to choose route, we hear (9)
I believe the answer is:
electrode
'conductor' is the definition.
(electrode is a kind of conductor)
'choose route we hear' is the wordplay.
'choose' becomes 'elect' (electing is a kind of choosing).
'route' becomes 'road' ('road' can be a synonym of 'route').
'we hear' shows a homophone (sound like).
'road' is a homophone of 'rode'.
'elect'+'rode'='ELECTRODE'
'to' is the link.
(Other definitions for electrode that I've seen before include "Conducting terminal" , "Conducting point by which electric current enters or leaves" , "Conductor for electricity as it enters or leaves" , "Conductor making contact in a circuit" , "Terminal of, eg, an acid battery" .)