Longman Dictionary of Contemporary English
police department
noun
EXAMPLES FROM CORPUS
▪ But the whole police department is in on it.
▪ I would guarantee him an exclusive, pictures and all, which is something he'd never get from the police department.
▪ In 1994, New Orleans hired a new police chief to rescue the corrupt, ineffective police department from itself.
▪ The Boca Raton police department gave him the name and phone number of a bank officer who could be reached in emergencies.
▪ There were some cases, nevertheless, of clear discrimination: age discrimination in the police department, for example.
▪ When anything like this happened, every office-holder in the community made speeches passing the buck on to the police department.
Wiktionary
police department
n. (context law enforcement English) police force, especially a local or regional branch of a national police force.
WordNet
police department
n. the department of local government concerned with enforcing the law and preventing crime