Just finished and submitted (leaving a blank at 13 across. Guardian website. More sighs)
Even though I found the answers, I just couldn't parse 8d and 12ac.
I loved 8 Down, I thought it was a clever clue. The 'flier' is something whose third letter drops four grades/moves down four letters alphabetically - the result is a rather old-fashioned word for 'excellent'.
Nearly there on this excellent puzzle, but drawing a blank on 15d. Unless I have gone wrong, I think the grid entry is s.n.c.e and the letter to be removed is a t.
You've got the right letters and the right removed letter. The solution to the clue is an anagram of one word in the clue with an 'e' for energy added. Hope this is of some help.