Incentive from drug carriers trading ecstasy for uranium - it's back to front (8)

I believe the answer is:

stimulus

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'incentive' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for stimulus that I've seen before include "That which produces a response" , "Motivating action" , "Prod or incentive" , "Something that rouses to action" , "Something promoting activity" .)

I've seen this clue in The Independent.

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