Darkroom solution (5)
I believe the answer is:
toner
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">darkroom solution</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 toner that I've seen before include "Lotion for cleansing the skin" , "Powder used in printers" , "Skin improver" , "Black or coloured powder used in a printer" , "Skin-cleansing lotion" .)