Insecticide used by riders (6)

I believe the answer is:

derris

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

'by riders' is the wordplay.
'riders' anagrams to 'DERRIS'.
Though, I am not clear how the anagram is indicated.

'used' is the link.

Can you help me to learn more?

(Other definitions for derris that I've seen before include "Woody plant" , "lethal powder" .)

I've seen this clue in the Evening Standard.

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