Perform extra work - a sonata (9)
I believe the answer is:
moonlight
This clue is a double definition.
'perform extra work' is the first definition.
'a sonata' is the second definition.
(moonlight sonata by beethoven)
(Other definitions for moonlight that I've seen before include "Ray" , "Steerer of boat" , "Do extra work" , "name for sonata" , "piano piece" .)