Underground, most of root vegetable (4)
I believe the answer is:
tube
'underground' is the definition.
(the London Underground is nicknamed the Tube)
'most of root vegetable' is the wordplay.
'most of' means to remove the last letter.
'root vegetable' becomes 'tuber' (I've seen this in another clue).
'tuber' with its last letter removed is 'TUBE'.
(Other definitions for tube that I've seen before include "London Underground or long cylinder" , "Pipe - railway" , "train network" , "appearing tinny?" , "Pipe; underground train" .)