Siemens will require silver for a conductor (7)
I believe the answer is:
sargent
'for a conductor' is the definition.
(I've seen this before)
'siemens will require silver' is the wordplay.
'siemens' becomes 'S' (synonyms).
'will require' is a charade indicator (letters next to each other).
'silver' becomes 'argent' (synonyms).
's'+'argent'='SARGENT'
(Other definitions for sargent that I've seen before include "John Singer --, US painter" , "Painter in America" , "artist" , "Conductor" , "American painter" .)