Some-one who acts like and wishes they are of a certain clique, but in truth arent, often they fill the more complex criteria of the clique, but miss the defining quality. Their character often seems forced.
A wannabe pimp will dress himself in flashy clothes, jewlery etc, will act cocky and de-meaning of females, like they own them...but in truth have never had a girlfriend, never been close to getting anywhere with a girl and probably never will.
A wannabe gansta will listen to 50 cent all day, dress in a rebelious hip-hop fashion and talk like their 'dope'. However they would probably cry to their mommies if they ever saw a gun and would be too scared to touch any form of drug or illegal good.
Look at those ghetto wannabes, they live in mansions for f*ck sake!