Transport network down? (4)
I believe the answer is:
tube
This could be a double definition.
'transport' is the first definition.
(the London Underground is nicknamed the Tube)
'network down?' is the second definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's an association between them I don't understand?
Can you help me to learn more?
(Other definitions for tube that I've seen before include "Can of beer (colloquial)" , "Cylindrical hollow conduit" , "Pack of toothpaste; type of train" , "London Underground or long cylinder" , "train network" .)