Paul Wall Fighting Drug Bust

TMZ spoke with Wall's rep ... who tells us cops entered the nightclub where Wall and fellow rapper Baby Bash had performed and told the rappers that their rented van smelled like marijuana.

The rep says cops searched the vehicle and all of the passengers and found "a filter of a joint, which tested positive for marijuana resin."

We're told Paul was charged with possession of marijuana under 2oz. in a drug free zone -- because they happened to be near a school.

The rep says Paul insists the weed wasn't his ... but adds, "This is truly the vendetta of a singular member of the El Paso County Constable Department and these facts will bear out over time."