Old rover loses right to supply narcotic drug (6)
I believe the answer is:
opiate
'narcotic drug' is the definition.
(I've seen this before)
'old rover loses right' is the wordplay.
I cannot really understand how this works, but
'old' could be 'o' (abbreviation) and 'o' is found in the answer.
'rover' could be 'i' (where the crossword setter is 'Rover') and 'i' is present in the answer.
The remaining letters 'pate' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'to supply' is the link.
Can you help me to learn more?
(Other definitions for opiate that I've seen before include "painkiller" , "Narcotic drug" , "one soporific" , "Drug from poppies" , "Does it ease pain" .)