Do nocturnal work on a sonata? (9)
I believe the answer is:
moonlight
'a sonata?' is the definition.
(moonlight sonata by beethoven)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for moonlight that I've seen before include "Work in a secondary job" , "Work late" , "do an extra job" , "Night-time illumination" , "Ray" .)