A mildly shocking drink? (3)
I believe the answer is:
jar
'a mildly shocking drink?' is the definition.
'jar' can be an answer for 'drink?' (colloquial). I am not certain of the 'a mildly shocking' bit.
'a mildly shocking drink?' is the wordplay.
'mildly shocking' becomes 'jr' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'drink?' means one lot of letters goes inside another.
'a' inserted within 'jr' is 'JAR'.
Can you help me to learn more?
(Other definitions for jar that I've seen before include "Cylindrical container as for jam" , "Glass of beer (colloq.)" , "(Jam) pot" , "Container; be incongruous" , "Pint glass of beer, perhaps" .)