In that example, 'a' usual is correct. The 'a' is used before a singular noun that begins with a consonant sound. I know . . . usual starts with the vowel 'u' but it has the consonant sound 'you' not the vowel sound 'u'. 'An' is usually used before nouns beginning with vowel sound. 'A' of course before nouns with a consonant sound.

I also agree that “a usual” is correct.

Terry's explanation is right on the mark. A usual activity but an unusual activity, for example.

An usual