Something caught, marked by X? (4)
I believe the answer is:
prey
'x?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for prey that I've seen before include "Predator's victim" , "Animal hunted by another for food" , "Quarry, target" , "Animal's victim" , "Animal hunted or caught for food" .)