the answer is the letter 's'.
The obvious word is "stellar." The transposition is "staller."
What is a seven letter word with w as the sixth letter?
evident
Stomach [side note: since this is a fairly trivial answer, what's the purpose of asking such obvious questions?]
Holiday is a seven letter word.
What is a seven letter word with o as the sixth letter
Luggage, is a seven letter word for bags.
evident
In my dictionary the seven letter word after fan is fanatic.
A seven letter word for religious dissenter is heretic.
Duchess is a seven letter word.
common