A has-been is someone who used to be important, famous, or very good at something, but that time has passed. People have moved on; the person is no longer in the centre of things, and sometimes they are half-forgotten. You hear it about singers, actors, sports stars, and now and then about politicians. It is informal, and it is not kind — calling someone a has-been puts them in the past on purpose. Gossip pages and sharp conversation use it; you would not put it in a polite letter or a job reference. It can also sit in front of a noun: a has-been actor. Though actually, people sometimes say it with sadness rather than spite: they fear becoming a has-been themselves. It looks like a verb, but in this use it is a noun (and sometimes an adjective). Write it with a hyphen.