Pipes; conduits (5)
I believe the answer is:
tubes
I believe this clue is a double definition.
'pipes' is the first definition.
(I know this)
'conduits' is the second definition.
(tube is a kind of conduit)
(Other definitions for tubes that I've seen before include "Long hollow cylinders" , "Piping" , "Hoses, pipes, lines etc" , "Hollow cylindrical shapes" , "Long hollow conduits" .)