The uppercase registers and the lowercase ones are actually the same.
When you yank using the lowercase version you replace the content of the register. When you yank using the uppercase version you append the content to the register.
Hence the standard answer is: Don't clear the register just use the lowercase version for the first yank.
Otherwise you can use the following call to set the register 'A' to empty string:
Or, alternatively (from @romainl)
Which starts recording the macro of register a (
qa) and immediately stop recording (
q), leaving it empty.
But to clear the 'A' register such that the
reg command don't list it (following @filbranden) you can:
:call setreg('a', )