From Longman Dictionary of Contemporary Englishin plain clothesin plain clothesSCPpolice officers in plain clothes are not wearing uniform → plain-clothes → plain
Examples from the Corpus
in plain clothes• They were in plain clothes and, in his opinion, drunk, arrogant and overpowering.• Large men in plain clothes and short haircuts had wanted to know his business.• Mr Montesinos was escorted by agents in plain clothes into a helicopter shortly after arriving at Lima airport.• In uniform and in plain clothes, roughly 50 were on different shifts asking each shopper for more information. 