17 down
One
sequence
of
notes
being
performed
in
which
person’s
unaccompanied
(9)
One sequence of notes being performed in which person’s unaccompanied (9)
hints
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checks
0
hints
The definition is at the end of the clue.
How the clue works
Look at in which person's unaccompanied — that's your definition. Then the wordplay builds it piece by piece: I (one), followed by SO, LA, and TI (a sequence of musical notes), and finally ON meaning being performed (as in "the show is on").