The word "retainer" used in rap vernacular often is referring to keeping a thug on hand that is willing to do
dirt. The
celebrity retains this person's services thru cash payment, or other bartering options such as weed or pussy in the event that a punch needs to be thrown (or otherwise) and the
celebrity would be open to a potential costly lawsuit, whereas the retainer does not have enough money worth be sued for, etc.
When Charlemagne Tha God was instigating Nelly during an interview, he was warned "Hey,man...
just say what you wanna say. We're all
men. I keep retainers
just for guys like you".