The dictionary entry also provides an example usage:
She had every right to be cross under the circumstances.
By plugging in the provided definition, we know this means the same thing as:
She had every right to be cross given the difficult nature of the situation.
Given the definite article there, we can assume we know what the situation is. The dictionary hasn’t provided it, so we must use our imagination, and imagine a situation that would make someone cross, such as:
She had only been working at the company just six months, and now they were closing their doors. She had moved here all the way from her hometown in hopes of starting a new career. She had loved her job, and enjoyed her coworkers. But now she would have to start looking for employment all over again. She had every right to be cross given the difficult nature of the situation.
She was getting ready to feed her baby just before bedtime, and realized the can of formula was empty. Oh, no! She dashed out of the house. The store was closing in five minutes. Maybe she could make it just in time. But as she got out of her car in the parking lot, she approached the door just in time to see the clerk locking the door. “Please,” she pleaded, “I just need to buy some formula for my baby.” She had hoped the baby in her arms might invoke sympathy from the clerk, but the clerk coldly replied, “Sorry, we’re closed.” Now she would have to drive all the way to the city to find a store that was still open. She had every right to be cross given the difficult nature of the situation.
That’s the way dictionaries work. They provide a short example usage, and you have to imagine how that would be used in a larger context.