Deep mud (4)
I believe the answer is:
mire
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">deep mud</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 mire that I've seen before include "Marsh" , "(Stretch of) clinging mud" , "Swampy ground" , "Bog - muck" , "Soft mud or dirt" .)