Perhaps armory the american version of armoury?
Morally becomes moralry if writing, one of the 3 "r" s replaves "1 L (student)", corrupt being the anagrind givung an "L" surplus.
I have got confused with 9d and 12a. 9d clue: Crystalline salt eroding dull old copper from bevel wheels. I have H??o?d. There will be an extra letter in wordplay. 12a intersects at end with second letter of 9d. Clue is generously feed with endless porridge. I have A?PL?
9 amply, endless porridge = Sam(p) + ply = feed giving "s" extra
12 hypoid but haven't parsed it yet
Don't do this crossword but do you have an indication of what the extra letter might be?