Instead of trapping it or killing it, I'd secure your coop and run to be absolutely certain the opossum can't get in to your chickens. Opossums are one of the VERY few predators of rats, and rats are just as deadly to your chickens and far, far more difficult to be rid of...if there's any chance you have rats around, let the opossum stay (albeit outside and away from your chickens) so to dine on rats.
We agree with RHS about not killing the possums. There is no way that a possum could get into our run or coop so we are OK with them being around because they do eat rats and we hate rats. I kill racoons and rats but leave almost everything else alone. A feral cat is much more likely to kill your chickens and they are also more likely to be able to get into a run and coop.
eggs are a good bait for possums anything spoiled or rotten is good as well, they pass around some nasty diseases especially around horses the folks here in my neighborhood do not worry about possums stealing their pets food