A down-to-earth part (4)
I believe the answer is:
root
'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" .)