I think Ive cracked it but don't quite see how! 5ac and 13d appear to be the 2 normal clues, the four unclued are defined by the extra letters and I have deduced the translation from the theme. Where I'm confused is how the extra letters confirm the interpretation and how the two normal clues form a cryptogram. I obviousl do not know what a cryptogram is, so any advice would be appreciated.