Chemical indicator (6)
I believe the answer is:
litmus
'chemical indicator' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for litmus that I've seen before include "Acid-alkali testing substance" , "Acid/alkali test dye" , "Substance used to distinguish between acid and alkali" , "Paper used to test for acid or alkali" , "Colouring material used as a rough acid-base indicator" .)