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' is an anagram indicator.
'many' becomes 'c' (C is the Roman numeral for 100).
'into' with letters rearranged gives '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" .)