Bath water unit
I believe the answer is:
litre
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">bath water unit</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 litre that I've seen before include "One cubic decimetre" , "Measure of liquid volume" , "Nearly two pints" , "Metric unit of volume" , "A pint or two" .)