Darkroom solution (5)
I believe the answer is:
toner
'darkroom solution' is the definition.
(I've seen this in another clue)
This is all the clue.
(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" .)