Type of test (6)
I believe the answer is:
litmus
'type 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 "It turns red with acids and blue with alkalies" , "'It turns red with acid, blue with alkaline (6)'" , "dye encountered in testing" , "Substance used to distinguish between acid and alkali" , "Substance turned red by acids" .)