Old pianist with cry of pain leaving Jack Sprat's wife? (4,6)

I believe the answer is:

fats waller

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'old pianist' is the definition.
(I know that Fats Waller was a jazz pianist)

I don't understand the remainder of the clue.

(Other definitions for fats waller that I've seen before include "American jazz musician, d. 1943" , "Jazzman" .)

I've seen this clue in The Guardian.

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