Conduit for a fluid (4)
I believe the answer is:
duct
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">conduit for a fluid</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for duct that I've seen before include "Tube for cables, air, etc" , "Air passage" , "Pipe conveying fluid or gas" , "The Eve of St - is a narrative poem by Keats" , "Tube for conveing cable, liquid etc" .)