The term 'CROOK'S COVER' is a combination of two crossword solutions: ALIAS (5 letters) and ALIBI (5 letters). An ALIAS is an assumed name used by someone to conceal their identity, while an ALIBI is an excuse or evidence used to prove that someone was elsewhere when a crime was committed. Therefore, 'CROOK'S COVER' is a phrase used to describe the methods used by criminals to hide their identity and provide an excuse for their whereabouts.