Competition: Brain flexing

Specifics

The rebel Cause lurking in the shadows is ready to reveal itself and strike at any moment now. In order to keep your grey matter active for such an event, you have been issued with a riddle to solve: What letter does '?' represent in this sequence:

?TTFF SSENT ETTFF SSENT

N.B. '?' does not represent the letter 'E'.

Platform

E-mail.

Comments

[Approved by House Summit].

Awards

5th level crescents for the first three to submit a correct answer.