Drug arrived concealed in hollow, with accompanying note (10)
I believe the answer is:
medicament
'drug' is the definition.
(I know that medicament is a type of drug)
'arrived concealed in hollow with accompanying note' is the wordplay.
'arrived' becomes 'came' (come can mean to appear or arrive).
'concealed in' indicates putting letters inside.
'hollow with' becomes 'dint' (I am not sure about this - if you are sure you should believe this answer much more).
'accompanying' says to put letters next to each other.
'note' becomes 'me' (musical note as in do-re-me).
'came' put inside 'dint' is 'dicament'.
'dicament' put after 'me' is 'MEDICAMENT'.
Can you help me to learn more?
(Other definitions for medicament that I've seen before include "drug?" , "Healing substance" , "Curative substance" .)