plain-clothes

From King Dictionary of Contemporary English
Related topics: Police
plain-clothesˌplain-ˈclothes adjective [only before noun] SCPplain-clothes police are police who wear ordinary clothes so that they can work without being recognized
Examples from the Corpus
plain-clothesShe started to strip off, all hell broke loose and he bolted while plain-clothes officers moved in to stage a cover-up.