Kill the returning Grand National horse (6)

I believe the answer is:

murder

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'kill' is the definition.
(I know that murder is a more specific form of the action kill)

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for murder that I've seen before include "a lot of crows" , "Homicide (very noisy when blue)" , "very difficult task" , "Serious crime (that may be blue?)" , "Unlawful premeditated killing" .)

I've seen this clue in The Irish Independent.

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