Bravo, Colleague! Not by any means a new theme (there is, for example, an Echo in a 1999 Listener puzzle), but handled here in a novel and entertaining way. Interesting that the theme words that weren't hidden at all, or hardly, were more difficult to spot than those that were. Reminded me of Poe's short story "The Purloined Letter", in which the letter was hidden in the letter rack.
What with a recent Spectator and now this, it is clearly time to buy the latest (19th) edition of Brewer's, of which I have not been an assiduous collector. I have an undated copy of the 1894 edition, the 8th (revised) edition of 1963, the 2nd (revised) edition of the Centenary edition (1981), the 15th (1995), and the Millennium edition (the 16th, 1999). The answer has been there since at least the 15th edition, listed under a headword that the preamble hints at; however, I found the information more easily in the comprehensive Wiki entry.