Weight of explosive power (7)

I believe the answer is:

megaton

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'weight' is the definition.
(megaton is a kind of weight)

I cannot understand how the rest of the clue works.

(Other definitions for megaton that I've seen before include "Unit of explosive power" , "Measure of explosive power - montage (anag)" , "Explosive power equivalent to one million tons of TNT" , "weight" , "it weighs a lot" .)

I've seen this clue in the Evening Standard.

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