Duct — channel (5)
I believe the answer is:
canal
I believe this is a double definition.
'duct' is the first definition.
(synonyms)
'channel' is the second definition.
(I know that channel can be written as canal)
(Other definitions for canal that I've seen before include "navigate this?" , "Long, narrow strip of water" , "Inland waterway" , "Duct" , "Grand Union perhaps" .)