Not just any old medicine? (8)

I believe the answer is:

specific

I'm a little stuck... Click here to teach me more about this clue!

'old medicine?' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

(Other definitions for specific that I've seen before include "Particular, definite" , "Clearly defined or identified" , "Explicit or in detail" , "Explicit, exact" , "Precise, clearly defined" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app