Spark off funny gagwriter, dropping a wisecrack at the opening (7)

I believe the answer is:

trigger

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'spark off' is the definition.
(I know that spark off can be written as trigger)

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

Can you help me to learn more?

(Other definitions for trigger that I've seen before include "Cause (device) to operate" , "Initiate" , "Lever on gun" , "Lever to activate firing mechanism" , "trip" .)

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

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