Prevent from succeeding (6)
I believe the answer is:
thwart
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">prevent from succeeding</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 thwart that I've seen before include "Frustrate and obstruct" , "cross" , "check" , "Hinder or prevent" , "Stymie" .)