Fidelio's Léonore, originally in disguise here: well, well, well! (9)

I believe the answer is:

oilfields

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'here well well well' is the definition.
The definition suggests a singular noun but the answer is a plural noun.

'fidelio's leonore originally in disguise' is the wordplay.
'originally' says to take the initial letters.
'in disguise' indicates anagramming the letters.
The first letter of 'leonore' is 'l'.
'fidelios'+'l'='fideliosl'
'fideliosl' anagrammed gives 'OILFIELDS'.

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I've seen this clue in The Guardian.

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