Horizontal bar (4)
I believe the answer is:
rail
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">horizontal bar</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 rail that I've seen before include "method of transport" , "Water bird; bar" , "Track for wheels" , "Supported bar - inveigh" , "Bar; waterside bird" .)