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".