WordNet sense 1 (a person secretly employed in espionage for a government):

secret agent, intelligence agent, intelligence officer

WordNet sense 2 (someone who can be employed as a detective to collect information):

pi, shamus, sherlock, private detective, private eye, private investigator

WordNet sense 3 (effective; producing a desired effect):

key

WordNet sense 4 (important in effect or meaning):

important, significant

WordNet sense 5 (relating to or requiring or amenable to treatment by surgery especially as opposed to medicine):

surgical

WordNet sense 6 ((of e.g. a machine) performing or capable of performing):

functional, running

WordNet sense 7 (serving to permit or facilitate further work or activity):

working

WordNet sense 8 (exerting force or influence):

effective