Indeed, candledave. When one has the two three-letter answers and the two either side of the unclued seven-letter answer, one can infer the second letter of that seven-letter answer, even without the theme, and as it doesn’t appear in the three-letter ones that rules it out of the bottom row. But it can’t be assumed a priori, and the clue beginning “What one may sign …” is one of the trickiest.