The term 'ACT THE RAT' can be broken down into three crossword solutions: GNAW (4 letters), DESERT (6 letters), and SQUEAL (6 letters). GNAW is a verb meaning to bite or chew on something persistently, usually with the teeth. DESERT is a noun referring to a dry, barren area of land with little vegetation. SQUEAL is a verb meaning to make a high-pitched cry or sound, often in response to pain or fear. Together, these three words form the phrase 'ACT THE RAT, ' which is an idiom meaning to betray someone or something.