Sea water pipe? (4)
I believe the answer is:
main
I believe this is a triple definition!
'sea' is the first definition.
(synonyms)
'water' is the second definition.
(main is a kind of water)
'pipe?' is the third definition.
(main is a kind of pipe)
(Other definitions for main that I've seen before include "Source of water" , "Chief in importance" , "Ocean or principal pipe" , "Chief, primary" , "Salt water" .)