Crossword Genius

Use envelopes surprisingly prepared in advance (2,4,6)

Ross

I believe the answer is:

up ones sleeve

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'prepared in advance' is the definition.
I don't know anything about this answer so I cannot judge whether this works.

'use envelopes surprisingly' is the wordplay.
'surprisingly' is an anagram indicator (a surprising ordering of the letters).
'use'+'envelopes'='useenvelopes'
'useenvelopes' anagrammed gives 'UP ONES SLEEVE'.

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