Thoroughgoing (7)
I believe the answer is:
radical
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">thoroughgoing</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for radical that I've seen before include "(One) favouring fundamental changes" , "stem" , "Innovative, progressive" , "'Revolutionary, extremist (7)'" , "Politically extreme" .)