Medicine lacking what it takes to be too nice (5)
I believe the answer is:
tonic
'medicine' is the definition.
(tonic is a kind of medicine)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for tonic that I've seen before include "Invigorating drink" , "makes you feel good?" , "Keynote of a scale" , "Gin mixer - pick-me-up" , "something medicinal" .)