Potato possibly cut underground (4)
I believe the answer is:
'underground' is the definition.
(the London Underground is nicknamed the Tube)
'potato possibly cut' is the wordplay.
'potato possibly' becomes 'tuber' (potato is an example of a tuber).
'cut' means to remove the last letter.
'tuber' with its last letter taken off is 'TUBE'.
(Other definitions for tube that I've seen before include "Underground transport", "Valve", "Pipe; underground train", "appearing tinny?", "Pipe (with trains?)".)