The "corpse in the copse" refers to the dead parachutist on the mountain. The parachutist is the "corpse" because he is dead, "a figure that hung with dangling limbs," when he lands on the island (95). The word "copse" refers to a small circle of trees.
In the beginning of chapter six, "Beast from the Air," a dead parachutist drifts onto the boys' island; his lines become twisted and tangled on the mountainside. When the wind blows and fills the parachute, the lines tighten, making the dead parachutist rise up as if he is looking around the island; then he bows back down when the strings and parachute relax.