Scorpius - because Sabre always appears to set his crosswords with such attention to detail, especially in the clue writing, it can be dangerous to think he has gotten away with something imprecise!
Regarding 29d, if you read through the entry for the word used as the seemingly incomplete definition, your misgivings may be allayed!
Staurlogist - you are correct. I too can remember Sabre using AA in BB to mean ABBA in the past. However, Jonathan Crowther's "A-Z of Crosswords" includes the following when discussing Sabre's project for putting the complete "oeuvres de Sabre" into electronic form.
"...he (Sabre) is suffering intense embarrassment at some of the the terrible cluing devices he has encountered. The use of 'in' as the imperative form of a transitive verb was not uncommon (so 'X in Y' would really mean 'Y in X'). Fortunately, he last used this device, which he now abhors, in 1982..."!! [my exclamation marks!
So it would seem that Sabre has seen some of the errors in his ways, and that you must have as long and as fond memories of Sabre puzzles as I do!!