Thanks, Arthur, for explanations. It works now via macro and I'm happy )

No, the letter doesn't exist at all in Unicode, and most likely won't be encoded in the future either, due to the rule known as "normalization stability". Generally speaking, you have to input it as Cyrillic a followed by combining acute accent, from the Unicode point of view. Inside TeX, you should of course build it with an accent macro.

Vyatcheslav