There is no single English word that meets the criteria. Here are some 7-letter words you can make if you repeat some of the given letters.
beerier, bittern, bittier, brevier, brinier, entente, enterer, innerve, interne, invitee, inviter, nervier, nervine, nettier, nitrite, nittier, reenter, reinter, rentier, retiree, retirer, reverer, reverie, reviver, ribbier, riveter, riviere, teenier, tentier, terreen, terrene, terrier, terrine, tinnier, veinier, vernier, vetiver, vintner, vitrine
trouble
Blaster
cherrub
There isn't one. You can make the 5 letter word bound
Some seven letter words starting with B are:bathtubdisturbperturbproverbrhubarbsuccumbtaxicabwashtub
Labours
because
inbound, unbound
Suburbs, Cobwebs, Hobnobs.
that word will be BE EASY.
bulwark
berserk?