Baffles, frustrates (5)
I believe the answer is:
foils
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">baffles</span>' is the first definition.<br/ ><span class="explanation-format__explanation">('foil' can be a synonym of 'baffle')</span><br/ ><br/ >'<span class="explanation-format__original">frustrates</span>' is the second definition.<br/ ><span class="explanation-format__explanation">('foil' can be a synonym of 'frustrate')</span><br/ ><br/ ></span>
(Another definition for foils that I've seen is " Thwarts with kinds of swords".)