The term 'USE A LETTER OPENER ON' can be summarized by two crossword solutions: SLIT and UNSEAL. SLIT is a four letter word meaning to make a long, narrow cut in something, usually with a sharp blade. UNSEAL is a six letter word meaning to remove a seal from something, such as a letter or envelope. Therefore, the term 'USE A LETTER OPENER ON' can be summarized as using a letter opener to make a long, narrow cut in something and to remove a seal from something.