An explosive? That's to underestimate it! (1-4)

I believe the answer is:

a-bomb

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'an explosive?' is the definition.
(I've seen this before)

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

(Other definitions for a-bomb that I've seen before include "explosive" , "deterrent" , "Killer" , "Nuclear weapon" , "Atomic weapon" .)

I've seen this clue in The Sun.

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