CREATE TABLE table_name_79 (time_retired VARCHAR(255), driver VARCHAR(255), laps VARCHAR(255), grid VARCHAR(255))
what is the time/retired when the laps is 6, the grid is less than 18 and the driver is clay regazzoni?
    
SQL: SELECT time_retired FROM table_name_79 WHERE laps = 6 AND grid < 18 AND driver = "clay regazzoni"