Hi Don
If you mean the one close to the stern,I would slice it off with a razor saw flush the where the deck will sit.Then drill a hole where it sits and one in the end of the piece you removed.Reglue back in position using a short length of abraded brass rod as a dowel in the holes you drilled and use epoxy not wood glue.I have done this in the past and the section of the bulkhead ends up stronger than the rest.You will have to clamp a length of planking across the bulkheads to clamp the loose piece to as the glue dries to hold it in line.
Kind Regards
Nigel