Work on one consumed by drug (6)
I believe the answer is:
opiate
'drug' is the definition.
(opiate is a kind of drug)
'work on one consumed' is the wordplay.
'work' becomes 'opie' (I can't explain this - if you can you should give a lot more credence to this answer).
'on' becomes 'at' (I've seen this in other clues).
'one consumed' means one lot of letters goes inside another.
'opie' placed around 'at' is 'OPIATE'.
'by' is the link.
Can you help me to learn more?
(Other definitions for opiate that I've seen before include "Drug such as morphine" , "Drug with an opium base" , "Type of drug" , "of heroin, perhaps" , "Poppy-based drug" .)