envelope
An envelope usually contains only one letter.
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)
Envelope.
FEEDBACK
Double-u
envelope
boldface, feedback
strength