Vocal tone of a prickly seed (4)

I believe the answer is:

burr

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'prickly seed' is the definition.
(I've seen this in another clue)

I don't understand the rest of the clue.

(Other definitions for burr that I've seen before include "Seed vessels with prickles" , "Former US vice-president - dialect way of pronouncing R" , "Soft regional accent" , "Rotary file - feature of some regional accents" , "Prickly, clinging seed-case" .)

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

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