Kind of test (6)
I believe the answer is:
'kind of test' 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 "Paper used to test for acid or alkali" , "dye encountered in testing" , "Substance used in test for acidity" , "Material from lichens used to test for acidity" , "Chemical tester" .)