Unfathomable (10)
I believe the answer is:
bottomless
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unfathomable</span>' is the definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for bottomless that I've seen before include "Very deep" , "Quince thus was in a jam" , "Inexhaustible" , "Having no limits" , "never getting used up?" .)