The term 'put on alert' is an expression that means to WARN or ROUSE someone. It can be broken down into three crossword solutions: ROUSE (5 letters), WARN (4 letters), and WARNED (6 letters). ROUSE means to wake someone up or to stir them into action. WARN means to give someone a cautionary warning about something. WARNED is the past tense of WARN, meaning to have given someone a warning. Put together, these three words form the phrase 'put on alert, ' which is used to describe the act of warning someone.