Master involuntary reaction with drug in vinegar (6,4)
I believe the answer is:
acetic acid
'vinegar' is the definition.
(vinegar contains acetic acid)
'master involuntary reaction with drug' is the wordplay.
'master' becomes 'ace'.
'involuntary reaction' becomes 'tic' (involuntary reflex of the body).
'with' says to put letters next to each other.
'drug' becomes 'acid' (acid is a kind of drug).
'ace'+'tic'+'acid'='ACETIC ACID'
'in' is the link.
(Other definitions for acetic acid that I've seen before include "pickling stuff?" , "Component of vinegar" .)