Hopeful? Initially getting off painkillers (7)

I believe the answer is:

aspirin

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'hopeful?' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for aspirin that I've seen before include "Headache pill" , "Common pain tablet" , "Headache killer?" , "Anti-inflammatory tablet" , "answer to pain?" .)

I've seen this clue in The Guardian.

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