Least dirty (8)
I believe the answer is:
cleanest
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">least dirty</span>' is the definition.<br/ ><span class="explanation-format__explanation">('clean' is the opposite of 'dirty' so 'cleanest' should be a synonym for 'least dirty')</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for cleanest that I've seen before include "Most dirt-free" , "Most hygienic" .)