Leading part of scathing jazz singing (4)

I believe the answer is:

scat

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'jazz singing' is the definition.
(I've seen this in another clue)

I don't understand how the rest of the clue works.

(Other definitions for scat that I've seen before include "Go away quickly" , "Wordless jazz singing" , "vocal style?" , "Shoo!" , "Dropping" .)

I've seen this clue in the Sydney Morning Herald.

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