Safety device in its own box (4)

I believe the answer is:

fuse

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'safety device' is the definition.
(electronic safety device)

I don't understand the remainder of the clue.

(Other definitions for fuse that I've seen before include "Integrate" , "Join, coalesce" , "'Blend, merge (4)'" , "Weld together through heat" , "Tube or cord for igniting an explosive" .)

I've seen this clue in the Evening Standard.

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