Mouth moistener (6)
I believe the answer is:
saliva
'mouth moistener' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for saliva that I've seen before include "Liquid that moistens the mouth" , "Fluid in the mouth" , "Mouth moisture, spittle" , "Spit" , "Clear liquid secreted into the mouth" .)