Stimulant drug — what one’s on when poor, mostly (5)

I believe the answer is:

upper

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'stimulant drug' is the definition.
(I know that upper is a type of stimulant drug)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for upper that I've seen before include "Amphetamine pill" , "Higher; superior" , "Higher in rank" , "Higher in status" , "Stimulant drug (informal)" .)

I've seen this clue in The Times.

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