Underground pipe (4)
I believe the answer is:
tube
I believe this clue is a double definition.
'underground' is the first definition.
('tube' can be a synonym of 'underground')
'pipe' is the second definition.
('tube' can be a synonym of 'pipe')
(Other definitions for tube that I've seen before include "Underground transport" , "channel" , "London Underground or long cylinder" , "train network" , "Pipe or hose" .)