It absorbs a third of mouth-watering sauce (4)
I believe the answer is:
ragu
'watering sauce' is the definition.
'ragu' can be an answer for 'sauce' (I've seen this before). I am not sure about the 'watering' bit.
'it absorbs a third of mouth' is the wordplay.
'it' becomes 'rg' (I can't explain this - if you can you should give a lot more credence to this answer).
'absorbs' indicates putting letters inside.
'third of mouth' becomes 'u' (3rd letter of 'mouth').
'rg' enclosing 'a' is 'rag'.
'rag'+'u'='RAGU'
Can you help me to learn more?
(Other definitions for ragu that I've seen before include "pasta sauce" , "tomato and meat sauce" .)