Large weight (7)
(Other definitions for megaton that I've seen before include "Unit of explosive power" , "it weighs a lot" , "Explosive power equivalent to one million tons of TNT" , "Powerful explosive unit" , "weight" .)
(Other definitions for ton that I've seen before include "Century" , "See 25" , "U nit of weight" , "Speed" , "A slangy 100mph - heavy going" .)