Instrument used by Purcell once (5)
I believe the answer is:
cello
'instrument' is the definition.
(I know that cello is a type of bowed stringed instrument)
'used by purcell once' is the wordplay.
'used by' indicates a hidden word.
'CELLO' is hidden within 'purcell once'.
(Other definitions for cello that I've seen before include "Large stringed instrument" , "Musical instrument like large violin held between knees" , "String quartet instrument" , "Orchestral instrument played between the legs" , "Bass stringed instrument" .)