Giving the sack when igniting (6)

I believe the answer is:

firing

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'giving' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for firing that I've seen before include "Shooting; sacking" , "Dismissing" , "Dismissal" , "Discharging a gun" , "Discharging, sacking" .)

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

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