Narrow waterway (6)
I believe the answer is:
strait
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">narrow waterway</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 strait that I've seen before include "Narrow sea-channel" , "Narrow waterway" , "Connecting passage of water" , "Sound" , "Difficult state for artist" .)