Plainclothes is an adjective used to describe a police officer who operates in civilian clothes. A participial adjective—plainclothesed or plainclothed—might seem to make logical sense, but plainclothes has functioned adjectivally for at least a century and is not questioned by anyone conversant with law-enforcement lingo. It’s closely related to plainclothesman, an obsolescent term for a plainclothes officer.
In current British publications, the term is hyphenated—plain-clothes—a little more often than it is unhyphenated. In U.S. publications, the unhyphenated form is far more common.
Of course, when plain is simply an adjective modifying clothes, whether in law enforcement or elsewhere, plain clothes is two words (see the last two examples below).
As two witnesses helped the man, the suspect fled on foot, the police said, until plainclothes officers reached him in a nearby driveway. [New York Times]
As police began to clear the area, plainclothes men with megaphones announced sales of the iPhone 4S wouldn’t start. [Wall Street Journal]
Uniformed and plain-clothes police clashed with pro-democracy protesters. [BBC]
At the time, Frewing was in plain clothes and was with two other plainclothes officers in dark area of Carrie Cates Court. [Vancouver Sun]
Eyewitnesses said a man in plain clothes approached an elderly lady who was trying to deliver a letter. [News.com.au]