1. a person or thing considered to be no longer of significance
that player/singer/actor is a has-been
A person who is considered a 'has been' is someone who was once famous or culturally significant but is no longer in the public eye or no longer popular or relevant. This is an expression you are more likely to hear in social contexts but it could be used to describe someone in the business community also. Similar expressions include ' a flash in the pan' or 'a one-hit-wonder'.