The burgler's contrivance was very well thought out, he had planned every single detail.
The criminal contrived a plan to escape from jail.
He could not contrive an explanation for his misbehavior.
When it has a subject, verb, punctuation, capitalization, a predicate, and a complete thought. A sentence is a sentence when it is handed down by a judge (or a magistrate).
Since he could not pass the exam, he contrived an excuse to leave the class.