Drainage system (5)
I believe the answer is:
sewer
'drainage system' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sewer that I've seen before include "Channel to carry away waste water and matter" , "Large pipe for carrying waste matter" , "Pipe for waste matter from houses, factories etc" , "Drainage channel" , "Main drain" .)