1. **State the problem:** We want to simplify the expression $1 - a$. 2. **Understand the expression:** This is a simple algebraic expression where 1 is a constant and $a$ is a variable. 3. **Simplify:** Since there are no like terms to combine, the expression remains as is. 4. **Final answer:** The simplified form of $1 - a$ is simply $1 - a$.