underachiever

n.

undecided, undeclared, undefended, undefined, undemanding, undemonstrative, undeniable, undeniably, under, under arms, under control, under the charge of, under the heel of, under the order of, under way, under-, under-secretary, under-sexed, under-side, underachieve, underact, underarm, underbelly, underbid, underbrush, undercarriage, undercharge, underclothes, undercoat, undercover, undercurrent, undercut, underdeveloped, underdog, underdone, underestimate, underexpose, underexposure, underfed, underfelt,