Crossword Genius

Familiar with duets, played by heart for proof (4,2)

Ross

I believe the answer is:

used to

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'familiar with' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

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(Other definitions for used to that I've seen before include "Familiar with" , "Not unfamiliar with" , "Was in the habit" , "Accustomed or inured" , "Did at times in the past" .)

I've seen this clue in The Independent.
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