Medicine is key (5)
I believe the answer is:
tonic
I believe this clue is a double definition.
'medicine' is the first definition.
(tonic is a kind of medicine)
'key' is the second definition.
(I've seen this before)
'is' is the link.
(Other definitions for tonic that I've seen before include "Quinine water" , "__water; gin mixer" , "shot in the arm" , "makes you feel good?" , "It goes with gin" .)