Not so dense (7)
I believe the answer is:
sparser
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not so dense</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for sparser that I've seen before include "Less thick" , "not so common" .)