An employee or other worker with some particular function or skill.
A spy, secret agent, or detective.
A participant in an operation.
Effectual or important.
He's usually in a good mood — the operative word there being "usually". Today was a disaster.
Functional, in working order.
Having the power of acting; hence, exerting force, physical or moral; active in the production of effects.
an operative motive
Producing the appropriate or designed effect; efficacious.
an operative dose, rule, or penalty
Based upon, or consisting of, a surgical operation or operations.
operative surgery