SQUEAL ON THE MOB is an expression that refers to someone who informs the authorities about criminal activities of a group or organization, usually in exchange for some kind of reward. The term can be broken down into two crossword solutions: SING (4 letters) and RAT (3 letters). SING means to produce a series of musical tones with the voice, while RAT is a derogatory term for someone who informs on another person. Together, these two words form the phrase SQUEAL ON THE MOB, which describes someone who provides information to the authorities about a criminal organization.