Laboratory substance (4)
(Other definitions for acid that I've seen before include "Corrosive liquid; drug" , "It is neutralized by alkali" , "So-called drug" , "Bitter or sour" , "Tart, sour" .)
'laboratory substance' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for chemical that I've seen before include "Like 4 or 24 dn" , "Laboratory substance" , "substance produced in a lab" , "Of a substance such as ethanol" , "Substance derived from or used in molecular reaction" .)