The acid test? (6)
I believe the answer is:
'the acid test?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for litmus that I've seen before include "This paper is used to test whether solution is acid or alkaline" , "It turns red with acids and blue with alkalies" , "Old dance" , "Chemical indicator" , "Substance used in test for acidity" .)