Thoroughgoing or extreme (7)
I believe the answer is:
radical
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">thoroughgoing</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">extreme</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ ></span>
(Other definitions for radical that I've seen before include "Drastic and thorough, as reforms may be" , "Basic" , "Fundamental - thoroughgoing" , "Sweeping - reformer" , "Fundamental - non-conformist" .)