Это полицейский слэнг. Это значит поймать болтливого человека на месте преступления с явными уликами и возможно в присутствии свидетелей.

Говорушка is formed from the verb говорить ''to speak/to talk'' and refers to a person who talks a lot. It isn't used too much. ''Поличным'' doesn't exist on its own, it always comes with the preposition ''с'' and the verbs поймать or взять (in English it would be ''to catch red-handed'', I believe)

To catch a criminal , who loves to chat too much , with the evidence of his guiltiness ..William , you seems to love books about cops ?

Говорушка is a made up word from говорить. The kind that you don't have to know to understand. Also it is something you'll never hear or use in an actual conversation.