Crossword Genius

What a composer produces for a long time? (5)

Ross

I believe the answer is:

opera

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'long time?' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'what a composer produces for a' is the wordplay.
'what a composer produces' becomes 'oper' (I can't justify this - if you can you should believe this answer much more).
'for' means one lot of letters go next to another.
'oper'+'a'='OPERA'

Can you help me to learn more?

(Other definitions for opera that I've seen before include "Drama or comedy set to music" , "Die Meistersinger is one" , "some theatrics" , "Musical 11 across" , ""The Magic Flute", for example" .)

I've seen this clue in the Evening Standard.
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