Narcotic prescribed but I ate other people's first (6)
I believe the answer is:
opiate
'narcotic' is the definition.
(I know that opiate is a type of narcotic)
'i ate other people's first' is the wordplay.
'other people' becomes 'op' (this could be a standard abbreviation which I don't know about).
'first' is a charade indicator (letters next to each other).
'i'+'ate'='iate'
'iate' put after 'op' is 'OPIATE'.
'prescribed but' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for opiate that I've seen before include "one soporific" , "Narcotic, such as morphine" , "description of religion?" , "Drug such as morphine" , "Drug from poppies" .)