LSD (4)
I believe the answer is:
acid
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">lsd</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for acid that I've seen before include "It is neutralised by alkali" , "Sour - type of rain or drop" , "Sour - LSD" , "Morose" , "See 4 Across" .)