19 across
12
keeping
horse
in
shape
(7)
12 keeping horse in shape (7)
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The definition is at the end of the clue.
How the clue works
Look at shape at the end — that's your definition, a nine-sided polygon. Now, "12" here refers to the answer to 12 across, which gives you NOON (as in 12 o'clock), and that contains NAG (horse) tucked inside it, giving you NO-NAG-ON.