One medicine you can change into many! (5)
I believe the answer is:
'one medicine' is the definition.
(tonic is a kind of medicine)
'change into many' is the wordplay.
'change' indicates an anagram.
'many' becomes 'c' (C is the Roman numeral for 100).
'into' with letters rearranged gives 'toni'.
'you can' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for tonic that I've seen before include "something invigorating" , "It goes with gin" , "do for musicians" , "Stimulus" , "something medicinal" .)