Conductor in a circuit (9)
I believe the answer is:
electrode
'conductor in a circuit' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for electrode that I've seen before include "Conductor making contact in a circuit" , "Conductor for electricity as it enters or leaves" , "Connecting conductor" , "Conducting terminal" , "Conducting point by which electric current enters or leaves" .)