Choose staff to join European conductor (9)
I believe the answer is:
electrode
'conductor' is the definition.
(I know that electrode is a type of conductor)
'choose staff to join european' is the wordplay.
'choose' becomes 'elect' (electing is a kind of choosing).
'staff' becomes 'rod' (both can mean a pole or stick).
'to join' means one lot of letters go next to another.
'european' becomes 'e' (abbreviation e.g. EU).
'elect'+'rod'+'e'='ELECTRODE'
(Other definitions for electrode that I've seen before include "Terminal of, eg, an acid battery" , "Conductor (eg, in battery)" , "Conducting contact" , "Connecting conductor" , "Conducting terminal" .)