Litmus reddener (4)
I believe the answer is:
acid
'litmus reddener' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for acid that I've seen before include "Drug LSD (colloq.)" , "Polluted precipitation" , "Etcher's fluid" , "Sour, sharp-tasting" , "something sharp" .)