Heighten, augment (7)
I believe the answer is:
enhance
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">heighten</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that enhance is a kind of heighten)</span><br/ ><br/ >'<span class="explanation-format__original">augment</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for enhance that I've seen before include "Intensify, increase or improve" , "improve things" , "Increase in power" , "Make better or more attractive" , "Improve the value of (something)" .)