Crossword Genius

The things they're told are incredible! (7)

Ross

I believe the answer is:

marines

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'the things' is the definition.
Although both the answer and definition are plural nouns, I can't see how they can define each other.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for marines that I've seen before include "Seminar for American troops" , "forces" , "Troops trained to serve on land or sea" , "Soldiers" , "Naval service" .)

I've seen this clue in The Irish News.
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