A down-to-earth part (4)

I believe the answer is:

root

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'part' is the definition.
(I know that root is a type of anatomical structure)

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for root that I've seen before include "Origin; eg, carrot" , "Underground growth" , "Solution of equation" , "Part of plant drawing nutrients from soil" , "Eg, turnip or carrot" .)

I've seen this clue in the Evening Standard.

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