Digestive fluid (4)
I believe the answer is:
bile
'digestive fluid' is the definition.
(bile is a kind of digestive fluid)
This is all the clue.
(Other definitions for bile that I've seen before include "Bitterness - digestive fluid stored in the gall bladder" , "Ill-temper" , "Peevish temper; a liver secretion" , "Gall bladder substance" , "Bitter fluid from the liver" .)