Thanks Brendan. New word on me too. My word fitted the definition but totally wrong. 3d should have got that especially as i thought that was the union meaning but thanks for the tip.
This is definitely not a difficult puzzle but it lulls you into thinking it is easy. But I really enjoyed Kite's misdirection and not obvious anagrams.
I'm joint 21d 13a COD too with perhaps a side order of 24a.
I'm stuck on last two, 21d (which people seem to like), and 24a. I can think of a word that fits my letters in 24, but can't parse it, and it would make 21 even more difficult!
I could post links, but they would be spoilers - try a Google search. It's a hard variety of chalk that can, at a pinch, be used for building (most chalk is too soft to be any use). Chambers is wrong!