Not shallow (4)
I believe the answer is:
deep
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not shallow</span>' is the definition.<br/ ><span class="explanation-format__explanation">('deep' is the opposite to 'shallow')</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for deep that I've seen before include "hard to understand" , "Extending far down" , "down a long way" , "Sunk low" , "Cavernous" .)