Time bomb failed to open vault (4)

I believe the answer is:

tomb

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'vault' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for tomb that I've seen before include "Mausoleum" , "Burial-vault" , "Underground vault" , "Large vault" , "Place for the burial of a corpse" .)

I've seen this clue in The Independent.

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