I don't remember ever coming across the spelling at 18ac, and it's not even given in the full-fat OED (not an uncommon situation, in reality). I remarked in a post not long ago that I felt that solvers taking on Azed without the aid of Chambers had a significant handicap (when parsing wordplays as much as when deducing the solutions). The versions for IOS and Android are very good and relatively inexpensive.
The first Azed of every month features a clue-writing competition, where solvers must produce their own cryptic clue for the entry marked with an asterisk (here the word at 12dn). Submitting a correctly completed grid is a pre-requisite, but Azed then awards the prizes based on his assessment of the quality of the clues. I would suggest that Chambers is essential for anyone hoping to succeed in the competition with any regularity.