“With 18-Down, classic 1967 song by a group whose name is a hint to solving the eight bonus clues (see note)”

Cry from a guard tower

Imaginary fiends

Assign a number, maybe

Takes orders at a restaurant

Great Plains tribe

"I'll be right with you"

Name that becomes a shape if you switch the second and fourth letters

They might leave you in stitches, for short

Not marbled, say