Thwart or frustrate (4)
I believe the answer is:
foil
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">thwart</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that thwart can be written as foil)</span><br/ ><br/ >'<span class="explanation-format__original">frustrate</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that frustrate can be written as foil)</span><br/ ><br/ ></span>
(Other definitions for foil that I've seen before include "Fencing weapon; outwit" , "Hinder - prevent" , "Prevent from succeeding" , "Frustrate; kitchen wrap" , "Frustrate with fencing sword" .)