operative

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Being in effect; having force; operating.
• adjective Functioning effectively; efficient.
• adjective Engaged in or concerned with physical or mechanical activity.
• adjective Of, relating to, or resulting from a surgical operation.
• adjective Significant; most important; key.
• noun A skilled worker, especially in industry.
• noun A secret agent; a spy.
• noun A private investigator.
• noun One who works for a political organization, often wielding influence out of public view.

from The Century Dictionary.

• noun One who operates in any line of business; an operator: used in certain circumstances as a convenient substitute for detective.
• Active in the production of effects or results; acting; exerting force or influence.
• Efficacious; effective; efficient.
• Concerned with the actual exercise of power, or the putting forth of effort or labor in the accomplishment of some end; practical.
• Of, pertaining to, or concerned with operations, as those of surgery.
• noun A workman; an artisan.

from the GNU version of the Collaborative International Dictionary of English.

• noun A skilled worker; an artisan; esp., one who operates a machine in a mill or manufactory.
• noun One who acts as an agent of another, especially a detective or spy.
• adjective Having the power of acting; hence, exerting force, physical or moral; active in the production of effects.
• adjective Producing the appropriate or designed effect; efficacious; effective.
• adjective (Surg.) Based upon, or consisting of, an operation or operations.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Effectual or important.
• adjective Functional, in working order.
• noun An employee or other worker with some particular function or skill.
• noun A spy, secret agent, or detective.
• noun A participant of an operation.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun someone who can be employed as a detective to collect information
• adjective effective; producing a desired effect
• adjective relating to or requiring or amenable to treatment by surgery especially as opposed to medicine
• noun a person secretly employed in espionage for a government
• adjective (of e.g. a machine) performing or capable of performing
• adjective being in force or having or exerting force