The word you're looking for is "envelope." It begins with the letter "e" and can contain just one letter inside it. This riddle plays on the dual meanings of "letter" as both a character in the alphabet and a written message.
A word that begins and ends with E and has only 1 letter in between is EYE. Begins and ends with E and the Y is in between.
E
strengths
Envelope
If the question means 'one other letter', I believe the answer is eye. (Adds one letter Y.) If the question is a riddle, the answer is 'envelope' as it begins and ends with 'e', and it contains a letter.
Envelope begins and ends with the letter e. It contains one letter.
One four letter word that starts with r is riot.
One-half is a hyphenated word. It begins with the letter O.
one is advocateannounce
An envelope
Envelope
Ergosterol, for one.