The term 'KEEP WATCH ON' can be broken down into three crossword solutions: SPY (3 letters), TEND (4 letters), and MONITOR (7 letters). SPY is a person who secretly collects information about an individual or organization, often for political purposes. TEND is to take care of or look after something. MONITOR is to observe and check the progress or quality of something over a period of time. Therefore, the phrase 'KEEP WATCH ON' can be interpreted as to secretly observe and check the progress or quality of something over a period of time.