Aargh! I like silex and mucros, but that neans I have 4D and / or 15 A wrong.
4D. Old harvesting tool, this cut around middle of cereals (5)
15A. Returning comments on school performance, head dropped e.g. irony.
I had sithe and trope, with the letter midway between h and p giving an l, but that would mean sithe having two wrong letters (apologies for anyone who doesn't have the full instructions)