Musical work, a rare and singular opera (4)

I believe the answer is:

opus

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'opera' is the definition.
(thesaurus)

'musical work a rare and singular' is the wordplay.
'musical work' becomes 'op' (abbreviation for opus).
'a rare' becomes 'u' (I can't justify this - if you can you should believe this answer much more).
'and' says to put letters next to each other.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'op'+'u'+'s'='OPUS'

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(Other definitions for opus that I've seen before include "Piece of music (with number)" , "Work or composition, usually musical" , "Major work" , "Creative work, eg in writing, art or music" , "Work (of a composer?)" .)

I've seen this clue in the Evening Standard.

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