Crossword Genius

A telling blow for the old coach (7)

Ross

I believe the answer is:

rattler

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This clue might be a double definition.

'a telling blow' is the first definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

'old coach' is the second definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps there's a link between them I don't understand?

'for the' acts as a link.

Can you help me to learn more?

(Other definitions for rattler that I've seen before include "I'm shaken!" , "Venomous snake" , "Excellent example" , "Rickety old vehicle" , "Nickname for US snake with horny segments in its tail" .)

I've seen this clue in the Sydney Morning Herald.
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