Pianist plays this essential sheet (8)

I believe the answer is:

keyboard

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'pianist plays this' is the definition.
The definition suggests a singular noun which matches the answer.

'essential sheet' is the wordplay.
'essential' becomes 'key' (key can mean essential or important).
'sheet' becomes 'board' (board is a kind of sheet).
'key'+'board'='KEYBOARD'

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(Other definitions for keyboard that I've seen before include "Data entry device" , "Computer input device" , "one lays hands on" , "facility for writing" , "way to enter data" .)

I've seen this clue in The Sun.

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