Most foul (7)
I believe the answer is:
rankest
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">most foul</span>' is the definition.<br/ ><span class="explanation-format__explanation">('rank' can be a synonym of 'foul')</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for rankest that I've seen before include "most objectionable" , "most offensive" .)