Potato possibly cut underground (4)
I believe the answer is:
tube
'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 "Pipe; underground train" , "appearing tinny?" , "Hollow cylindrical object" , "Cylindrical hollow conduit" , "Long, hollow cylinder" .)