Detonated then left (3,3)

I believe the answer is:

set off

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If this is the answer I cannot explain how the clue works.

(Other definitions for set off that I've seen before include "Depart" , "Begin journey or explode bomb" , "Counterbalance" , "Begin journey or cause to ignite" , "'Leave on a journey (3,3)'" .)

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

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