Conductor's chosen baton to lead first part of Eroica (9)
I believe the answer is:
electrode
'conductor's' is the definition.
(electrode is a kind of conductor)
'chosen baton to lead first part of eroica' is the wordplay.
'chosen' becomes 'elect' ('elect' as an adjective can mean chosen).
'baton' becomes 'rod' (I've seen this before).
'to lead' says to put letters next to each other.
'first part of' suggests taking the first letters.
The initial letter of 'eroica' is 'e'.
'elect'+'rod'+'e'='ELECTRODE'
(Other definitions for electrode that I've seen before include "Conductor for electricity as it enters or leaves" , "Conductor making contact in a circuit" , "Conductor (eg, in battery)" , "Conducting terminal" , "Connecting conductor" .)