Synonyms for Has-been:

adj.

behind the times (adjective)
back-number.

n.

lightweight, minion, small potatoes, nonentity, mediocrity, minnow, nobody, small fry, lackey. failure (noun)
underachiever, born loser, also-ran, dud, flop, loser, nobody, turkey.
has-been (noun)
back-number.
inferior (noun)
second fiddle, second, loser, flop, underdog, born loser, also-ran, subordinate, nonstarter, reject, lemon, turkey, second banana, underling, subaltern, underachiever, failure, dud, inferior.
loser (noun)
failure, underdog.

Other synonyms:

minion, nonentity. nobody, minnow, small fry, mediocrity. lightweight. Other relevant words:
minnow, minion, nobody, back-number, small fry, lightweight.