The things they're told are incredible! (7)
I believe the answer is:
marines
'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" .)