Crossword Genius

Drug a German injected into Morse? (7)

Ross

I believe the answer is:

codeine

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'drug' is the definition.
(codeine is a kind of drug)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for codeine that I've seen before include "Niece, do take this drug" , "Common opium-based tablet used to relieve pain" , "Mild narcotic used as a pain-killer" , "Derivative of opium used as an analgesic" , "Painkiller derived from opium" .)

I've seen this clue in The Sun and The Independent.
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