(BTW some people actually do want to eliminate the police)
"Defund the police" in the 'mainstream' such as there is one, means to reduce funding for law enforcement and shift that funding to public health, social workers, and community programs, in a way that reduces the scope of calls police will get, and reduce the number of calls police are required to respond to.
Police Unions are not publicly funded, so defunding them doesn't make sense. Renegotiating collective bargaining agreements with unions does. "Smash Police Unions" has a nice ring to it.