A former Borg drone was a Borg drone which was severed from the collective.
In 2375, The Doctor assured Seven of Nine, who was experiencing "the Borg equivalent of multiple personality disorder", that she was "a Borg drone who's now becoming an individual. That's practically unheard of. There's bound to be rough spots along the way. We just have to get past them." (VOY: "Survival Instinct")
Bjayzl, a criminal businesswoman based on Freecloud, was known to butcher ex-Borg for parts, and was wanted by the Fenris Rangers for these practices. Among Bjayzl's victims was Icheb, whose death was particularly jarring for Seven of Nine. (PIC: "Stardust City Rag")