12 across
Mortified,
having
to
sing
wordlessly
to
one
particular
backing
(10)
Mortified, having to sing wordlessly to one particular backing (10)
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The definition is at the beginning of the clue.
How the clue works
The definition is Mortified — deeply embarrassed. For the wordplay, start with HUM (to sing wordlessly), then I (one), and finally DETAIL (particular) reversed thanks to "backing" to give LIATED.