Beethoven sonata? Perform other work (9)
I believe the answer is:
moonlight
I believe this clue is a double definition.
'beethoven sonata?' is the first definition.
(I've seen this before)
'perform other work' is the second definition.
(to moonlight is to do a second job)
(Other definitions for moonlight that I've seen before include "Hold a second job" , "Lunar rays seen in thin gloom" , "Burn the midnight oil" , "Secretly work" , "Night-time illumination" .)