The most common and most likely answer to this crossword clue is the 8 letter word KILOTONS
I've seen this crossword clue in The Guardian
K
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Buddy explains!
The straight part
We know that "They measure explosions" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they measure explosions = kilotons.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 KILOTONS mean?
KILOTONS 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.