The clues are in alphabetical order and their length is not given 4. Odd figure lost before delayed payment ...starts with A 22. Begin to work after being turned on .... starts with I /J/K/L/M/N/O 23....
From the answer to each across clue one letter must be omitted wherever it occurs (often more than once) before entry. To the answer to each down clue one letter must be added, at any point but not...
Some clues have a misprint: 9a Bank account stops providing support (4) BA?? 10a Snorting grams, debauched guy on date was a wrong 'un (6) ??E??E 29a Killer reportedly more revered in the past (6)...
21d Holds weapon immobilising devices. .... number of letters not known
A "themed person" is then removed from the answer above to leave A?MS for the grid entry (not necessarily a real word)...