Crossword Genius

Concert instrument that's flat (5,5)

Ross

I believe the answer is:

grand piano

I'm a little stuck... Click here to teach me more about this clue!

This clue could be a double definition.

'concert instrument' is the first definition.
(I've seen this in another clue)

'flat' is the second definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe there's an association between them I don't understand?

'that's' acts as a link.

Can you help me to learn more?

(Other definitions for grand piano that I've seen before include "Large stringed instrument" , "Keyboard instrument" , "one getting tickled?" , "Large instrument with horizontal strings" , "responsible for producing notes ..." .)

I've seen this clue in The Telegraph.
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