Catch a crook is a phrase used to describe the job of law enforcement officers, who are tasked with apprehending criminals and bringing them to justice. The crossword solutions for this phrase are NAB (3 letters) and NAIL (4 letters). NAB is an acronym for 'Nabbing a Bad Guy, ' which is a slang term for arresting a criminal. NAIL is a verb meaning to capture or apprehend someone. Both terms refer to the same action of apprehending a criminal.