A detective who is a consultant; a private investigator(noun)
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Use "consulting detective" in a sentence
"So when he came into his hotel room and found his young wife huddled inertly beneath the bed-clothes, he picked up the telephone to summon tea and biscuits, administered a quick dose of contraband brandy, and then proceeded to an alternative not generally permitted a consulting detective when faced with a distressed client: He bundled Russell into the bath, undergarments and all, and turned the taps on hot and full."