The most common and most likely answer to this crossword clue is the 7 letter word KILOTON
K
I
L
O
T
O
N
More clues leading to the result KILOTON
Buddy explains!

The straight part
We know that "massive weight" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case massive weight = kiloton.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does KILOTON mean?
KILOTON noun- A measure of the strength of an explosion or a bomb based on how many thousand tons of TNT would be needed to produce the same energy.