Depraved sort Royal Engineers will copy? (9)

I believe the answer is:

reprobate

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'depraved' is the definition.
(I know that depraved can be written as reprobate)

I can't explain the rest of the clue.

(Other definitions for reprobate that I've seen before include "Person without moral scruples" , "Disapprove of" , "Villain" , "Berate pro, being an unprincipled person" , "Be a porter for an unprincipled person" .)

I've seen this clue in The Telegraph.

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