spoonsful
That's puzzling.
voldemort
The eight-letter word you are looking for is "envelope." It contains one letter (as in a piece of mail) inside it, while also being eight letters long.
The word possibly is an 8-letter word that contains the letter B.
Clippers (LA)
envelopeAn envelope usually contains only one letter.
FEEDBACK
boldface, feedback
therein
stir
dessert
Inimical comes to mind, but I'm sure there are others. I can do you one letter better: a 7 letter word that contains three Is is "idiotic".