Although I have the only word which fits in 13D, I'm having trouble parsing the clue:
13D 5's alternative regimen: one of 5's 2? (8)
5D is a torus shaped cake covered in sugar (8 letters), 2D is a multi coloured arch of light in the sky (7 letters) and the answer for 13D is another sort of sweet made of beaten egg-whites and sugar.