Complain bitterly (4)
I believe the answer is:
rail
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">complain bitterly</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 rail that I've seen before include "Track - complain - bird" , "Supported bar - inveigh" , "Scold, berate" , "Bar - marsh bird" , "Water bird; bar" .)