Fortify, buttress (10)
I believe the answer is:
strengthen
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">fortify</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >'<span class="explanation-format__original">buttress</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ ></span>
(Other definitions for strengthen that I've seen before include "Reinforce, substantiate" , "Bolster" , "Build up" , "Make more powerful" , "Make tough" .)