It translates as "Sixteen Solutions must be enciphered before entry in the grid. One of these entries is initially an abbreviation"
As LIK says it was a one letter shift. However the actual crossword enciphering is not always a one letter shift. Each of the sixteen enciphered answers always include a "shift", but the mumbers of letters shifted are different for each entry.
You need to solve the clue first and don't enter it until you have a 'Crosser". Then compare the letters that intersect, and work out how many letters there are between them, and that is the number that you must shift each letter for the enciphered answer. You may for example, have to shift all the letters of one answer by six letters and the next answer that needs to be enciphered all by ten letters or any other number.