Etcher's fluid (4)
I believe the answer is:
acid
'etcher's fluid' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for acid that I've seen before include "It is neutralised by alkali" , "Caustic and biting" , "Biting - tart" , "Type of chemical" , "Sour - type of rain or drop" .)