Digestive juice (4)
(Other definitions for acid that I've seen before include "Opposite of alkali; sour" , "It is neutralized by alkali" , "So-called drug" , "- - -drop, sweet" , "Sarcastic - tart" .)
'digestive juice' is the definition.
(bile is a kind of digestive juice)
This is the entire clue.
(Other definitions for bile that I've seen before include "Spleen" , "Bitter fluid from the liver" , "Ill-temper" , "Anger, bitterness" , "Digestive juice secreted by the liver" .)