Correct spelling, explanation: many words in English come from other languages. It is no different in the case of the word reverend, which comes from French and Latin. If you want to know how to spell it correctly, just look at the etymology. We can see three of the same vowels in this noun/adjective. It would be a mistake to think that the last -e should be replaced with -a. Reverand is not correct.

Definition of reverend:
1. noun, a title for a member of the Christian clergy
The service was led by Reverend Connelly.
When I walked into the church, our main reverend was praying.
2. adjective, worthy to be revered
I think your grandfather is a reverend gentleman.
I want to become a member of the church and be reverend by others.

Collocations with reverend:
Some most commonly used collocations include: the Right Reverend.
The Right Reverend is used as a title for high ecclesiastical officials.

Incorrect spelling, explanation: when spelling the word reverend, we should look at its etymology. It is a borrowing from Middle French révérend, as well as from the Latin future passive participle reverendus. There is only one vowel in this word: -e. There is no place for the letter -a here. That’s why reverand is just a mistake.