Assistant dropping unknown from lead of Mozart opera (4)
I believe the answer is:
aide
'assistant' is the definition.
(aide is a kind of assistant)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for aide that I've seen before include "auxiliary" , "Confidential assistant or advisor" , "someone in assistance" , "Assistant to a more senior person" , "Helper" .)