Scotch diluter (4)
I believe the answer is:
soda
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">scotch diluter</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for soda that I've seen before include "Variety of pop" , "fizz" , "pressurised liquid" , "Type of bread, water or fountain" , "Whisky mixer" .)