Do nocturnal work on a sonata? (9)
I believe the answer is:
moonlight
'a sonata?' is the definition.
(moonlight sonata by beethoven)
'do nocturnal work on' is the wordplay.
I cannot really understand how this works, but
'on' is within the answer.
This may be the basis of the clue (or it may be nonsense).
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" .)