How does this piece of wargear work in 6th edition when not in a Challenge?
In a challenge, it is pretty clear how it works. However, outside of a challenge, you don't allocate hits to models, only to units. So:
A) Right before the wound rolls, declare that your Clone Field wearer will take the wounds, and negate however many you rolled for.
B) After the wound rolls, declare that your Clone Field wearer will take the wounds, and negate however many you rolled for.
C) You're out of luck, and the Clone Field is useless.
And before you say "You should just stick with the Shadowfield", I'm more specifically talking about Urien.