Depression is necessary for effective use of the keys it holds (5)

I believe the answer is:

piano

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'it holds' is the definition.
The definition suggests a singular noun which matches the answer.

I cannot understand how the rest of the clue works.

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(Other definitions for piano that I've seen before include "Musical instrument - softly, please" , "Opposite of forte" , "Clavier" , "one of 6 [STEINWAYS]" , "key holder" .)

I've seen this clue in The Irish News.

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