We don't have synonyms for plain-clothes yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are plain clothes and plainclothes.
(adj) :
Alternative form of plainclothes (adjective) [(usually of a police officer) Wearing ordinary civilian clothes instead of a uniform, in order to avoid detection.]
(adj) :
Alternative spelling of plain clothes when used attributively.
(adj) :
Alternative form of plainclothes (adjective) [(usually of a police officer) Wearing ordinary civilian clothes instead of a uniform, in order to avoid detection.]
(adj) :
Alternative spelling of plain clothes when used attributively.