South American neighbours, one heartless and one short, sent back copy (9)

I believe the answer is:

reproduce

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

I don't understand the remainder of the clue.

(Other definitions for reproduce that I've seen before include "Ape" , "To propagate" , "[DOUBLE]" , "Make a child" , "Make a copy of (something)" .)

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