To be too inquisitive about a medicine ingredient can be costly (5)
I believe the answer is:
pricy
'be costly' is the definition.
(synonyms)
'to be too inquisitive about a medicine ingredient' is the wordplay.
'to be too inquisitive' becomes 'pry' (I've seen this in another clue).
'about' is an insertion indicator.
'a medicine ingredient' becomes 'ic' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'pry' enclosing 'ic' is 'PRICY'.
'can' acts as a link.
Can you help me to learn more?
(Other definitions for pricy that I've seen before include "Expensive" , "dear!" .)