Familiar with duets, played by heart for proof (4,2)
I believe the answer is:
used to
'familiar with' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(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" .)