In a dancing motion or while dancing; in a state of dancing.
From Middle English 'a-' (on/in) + 'dance,' where 'a-' was a productive prefix creating adverbial phrases. The form is archaic and poetic, from Old English directional prefix patterns.
You'll mostly find 'adance' in poetry—like in Tennyson who wrote about flowers 'adance' in wind. It's from an old English pattern of putting 'a-' in front of words to show continuous action, like 'afire' or 'asleep,' but most of these have vanished from modern English.
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