Way to get irrational number's root (6)

I believe the answer is:

turnip

'root' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for turnip that I've seen before include "'Root vegetable, yellow or white (6)'" , "Rip nut up for a vegetable" , "Plant with round white root used as vegetable" , "Globular food plant" , "Watch" .)

I've seen this clue in The Independent.

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