Water pipe (4)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">water pipe</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for main that I've seen before include "Principal or high seas" , "Chief in importance" , "Salt water" , "Chief, primary" , "Ocean or principal pipe" .)
(Another definition for bong that I've seen is " pipe".)