Particular drug (8)
I believe the answer is:
specific
I believe this is a double definition.
'particular' is the first definition.
('specific' can be similar in meaning to 'particular')
'drug' is the second definition.
(specific is a kind of drug)
(Other definitions for specific that I've seen before include "Definitive or exact" , "Particular, special" , "Old medicine" , "medieval drug" , "Well-defined" .)