Far down (4)
I believe the answer is:
deep
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">far down</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for deep that I've seen before include "Sunk low" , "hard to understand" , "(Of sound) low in pitch" , "Heartfelt" , "profoundly" .)