Still struggling with last two from Saturday's puzzle.
15a. "Piece of chalk for a pupil, a destroyer" (6). I've got -L-A-- and can only see alkala, the chemical powder, but may be on entirely wrong track
Also, 17d "Fixer at first turning up next to 18 down"(7). I've got --A-I-N and 18d, plus if alkala is right the first letter could be a; but then I can only get "alation" ??
Think I'm up the wrong tree so any help welcomed