Explains tantrum could break out at any moment (7)

I believe the answer is:

instant

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'moment' is the definition.
(I know that moment can be written as instant)

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

Can you help me to learn more?

(Other definitions for instant that I've seen before include "type of coffee" , "Happening immediately" , "Tick" , "Immediate, twinkling" , "Occurring immediately, like some fast food" .)

I've seen this clue in The Irish Times.

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