Are they for opening the piano? (4)

I believe the answer is:

keys

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'are they' is the definition.
The definition suggests a plural noun which matches the answer.

I don't understand the remainder of the clue.

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(Other definitions for keys that I've seen before include "House of ----, IoM legislature" , "A to G perhaps" , "Lock openers" , "Openers - low-lying islands" , "Islands; piano parts" .)

I've seen this clue in the Evening Standard.

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