Underground root primarily -- e.g. a potato (5)
I believe the answer is:
tuber
'potato' is the definition.
(potato is an example of a tuber)
'underground root primarily' is the wordplay.
'underground' becomes 'tube' (the London Underground is nicknamed the Tube).
'primarily' indicates taking the first letters.
The first letter of 'root' is 'r'.
'tube'+'r'='TUBER'
'e g a' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for tuber that I've seen before include "A brute of a spud" , "One goes underground" , "or still vegetating?" , "Fleshy underground stem (eg of the potato)" , "A potato, for instance" .)