The underground pipe (4)
I believe the answer is:
tube
I believe this clue is a double definition.
'the underground' is the first definition.
(the London Underground is nicknamed the Tube)
'pipe' is the second definition.
(I know that pipe can be written as tube)
(Other definitions for tube that I've seen before include "Pipe or hose" , "Hollow body for the underground" , "train network" , "Pipe; underground train" , "Cylindrical hollow conduit" .)