Composer opens Buenos Aires City Hall (4)

I believe the answer is:

bach

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'composer' is the definition.
(Bach is an example)

I can't explain the rest of the clue.

(Other definitions for bach that I've seen before include "JS --, baroque composer" , "dear to the Welsh" , "Johann Sebastian . . . . , great composer (4)" , "musical great" , "Relating to stars" .)

I've seen this clue in the Irish Times.

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