in plain clothes
The police officer was in plain clothes.
Tom vendeu o carro que ele tinha roubado para um policial à paisana.
Tom sold the car he had stolen to an undercover police officer.
Há dois à paisana, dois carros, quatro do FBI.
There are two plainclothes officers, two cars, four FBI agents.
— O Peso de um Passado (1988)
Ele me olhou como se fosse um oficial à paisana!
He looked at me like I was an officer in plain clothes!
— O Conformista (1971)