In extremes of scarlatina, not quite breathing spit (6)
I believe the answer is:
saliva
'spit' is the definition.
(synonyms)
I can't explain the remainder of the clue.
(Other definitions for saliva that I've seen before include "Mouth secretion" , "Liquid that moistens the mouth" , "Spit" , "Fluid in the mouth" , "Mouth moisture, spittle" .)