A nervous young woman went to the police station and made a complaint. An old policeman comforted her. Then she relaxed a little, but still "was wrapped tight and ticking ..."
What does "ticking" mean here? and what does "this one" refer to?
Single White Female
“My name is Allison Jones, and I live at One Seventy-two West Seventy-fourth Street.”
He smiled. “And you sound like a very nice and well-prepared twelve-year-old reciting in front of the class. Relax, Miss Jones. Like the PR ads say, your police department cares. This old cop does, anyway.”
“Not so old,” she said, smiling back as the tension loosened its grip on her. The set of her shoulders changed beneath the blue coat, became less squared and then slumped wearily. But the rigid cast of her jaw and mouth remained grim. She was wrapped tight and ticking, this one.