Underground root, primarily? (5)
I believe the answer is:
tuber
'underground root primarily?' is the definition.
I can't judge whether this defines the answer.
'underground root primarily?' is the wordplay.
'underground' becomes 'tube' (the London Underground is nicknamed the Tube).
'primarily?' indicates taking the first letters (I've seen 'primarily' mean this).
The initial letter of 'root' is 'r'.
'tube'+'r'='TUBER'
Can you help me to learn more?
(Other definitions for tuber that I've seen before include "Thickened underground stem" , "Root of potato etc" , "Rhizome, as of potato" , "Potato maybe" , "Root eg of potato" .)