10 across
Great
opera
houses
going
under?
The
opposite
(4)
Great opera houses going under? The opposite (4)
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The definition is at the end of the clue.
How the clue works
Look at going under? The opposite — that's your definition, since the opposite of underneath is ATOP (on top). The answer is hidden inside "Great opera" — gre-AT OP-era — with "houses" doing the job of signalling that the word is concealed within.