Watery substance produced in the parotid, sublingual and submandibular glands (6)

I believe the answer is:

saliva

(Other definitions for saliva that I've seen before include "Liquid that moistens the mouth" , "Mouth moisture, spittle" , "Mouth secretion" , "Clear liquid secreted into the mouth" , "Fluid in the mouth" .)

I've seen this clue in The Independent.

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