Sour, tart (6)
I believe the answer is:
acidic
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">sour</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ >'<span class="explanation-format__original">tart</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ ></span>
(Other definitions for acidic that I've seen before include "Vinegary" , "Sour; bitter" , "Sarcastic or sour" , "Containing a sour-tasting substance" , "that leaves a bitter taste" .)