Opening through which fluid can pass (5)
I believe the answer is:
inlet
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">opening through which fluid can pass</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for inlet that I've seen before include "Small arm of the sea" , "Coastal bay" , "Place of entry" , "Entry; arm of the sea" , "Small area of sea" .)