Potato stem found by Underground user? (5)
I believe the answer is:
tuber
'potato stem' is the definition.
(I've seen this before)
'by underground user?' is the wordplay.
'by' says to put letters next to each other.
'underground' becomes 'tube' (the London Underground is nicknamed the Tube).
'user?' becomes 'r' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'tube' put next to 'r' is 'TUBER'.
'found' acts as a link.
Can you help me to learn more?
(Other definitions for tuber that I've seen before include "Fleshy root, vegetable say" , "Rhizome, as of potato" , "Fleshy underground root" , "Root eg of potato" , "Potato, for example" .)