One medicine you can change into many! (5)
I believe the answer is:
tonic
'one medicine' is the definition.
(tonic is a kind of medicine)
'can change into many' is the wordplay.
'can change' indicates an anagram.
'many' becomes 'c' (C is the Roman numeral for 100).
'into' is an anagram of 'toni'.
'toni'+'c'='TONIC'
'you' acts as a link.
(Other definitions for tonic that I've seen before include "something invigorating" , "It goes with gin" , "do for musicians" , "Stimulus" , "something medicinal" .)