Acid indicator (6)
I believe the answer is:
litmus
'acid 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 "'It turns red with acid, blue with alkaline (6)'" , "Chemical tester" , "It turns red in acid and blue in alkali" , "Material from lichens used to test for acidity" , "Substance used to tell acid from alkali" .)