Instrument used by Charpentier (4)
I believe the answer is:
harp
'instrument' is the definition.
(harp is a kind of instrument)
'used by charpentier' is the wordplay.
'used by' indicates the answer is hidden within the clue.
'HARP' can be found hidden inside 'charpentier'.
(Other definitions for harp that I've seen before include "Stick to one theme" , "''The ... that once through Tara's halls'' (4)" , "Instrument for angel" , "Heavenly instrument?" , "Stringed instrument" .)