Pipe for sea water? (4)
I believe the answer is:
I believe this is a triple definition!
'pipe' is the first definition.
(main is a kind of pipe)
'for sea' is the second definition.
(synonyms)
'water?' is the third definition.
(main is a kind of water)
(Other definitions for main that I've seen before include "Ocean or principal pipe" , "source of gas" , "Chief; sea; pipe" , "Principal motivation" , "river in Germany" .)