Break down or fail (7)
I believe the answer is:
founder
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">break down</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >'<span class="explanation-format__original">fail</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(foundering is a kind of failing)</span><br/ ><br/ ></span>
(Other definitions for founder that I've seen before include "Beginner" , "(Of ship) fill with water and sink" , "One who makes an institution" , "Sink like originator?" , "One sets up firm" .)