Duct (4)
'duct' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for pipe that I've seen before include "Water carrier" , "Wine cask; sing shrilly" , "Something in the organ loft" , "Whistle" , "Tube; wind instrument" .)
(Other definitions for tube that I've seen before include "Underground transport" , "London Underground or long cylinder" , "Can of beer (colloquial)" , "Pipe - railway" , "Pipe (with trains?)" .)