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