Fire unlimited ammunition, you see -- that'll get 'em jumping! (4,4)

I believe the answer is:

sack race

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'you see that'll get 'em jumping' is the definition.
I can't tell whether this definition defines the answer.

'fire unlimited ammunition' is the wordplay.
'fire' becomes 'sack' (dismiss someone from their job).
'unlimited' says to take the centre (discard the edges or 'limits' of the word).
'ammunition' becomes 'tracer' (tracer is a kind of ammunition).
The middle letters of 'tracer' are 'race'.
'sack'+'race'='SACK-RACE'

Can you help me to learn more?

(Other definitions for sack race that I've seen before include "Jumping contest" , "'Novelty competition where entrants run with their feet in a bag (4,4)'" , "Get rid of nation" , "event - with 10d" , "athletics contest" .)

I've seen this clue in The Telegraph.

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