Hi Marca,
I'm in the very early stages so could be wrong. But I have a hunch.
From 15ac - casein being a protein, giving case - if s is replaced with adjacent letters (r,t) it gives a word.
Same for 18a, which is hidden in the clue for the 5 letter answer, and replacing the middle with two adjecent letters gives a word.
And same for 13ac, a simple anagram.
Hoever the song isn't the Australian sitcom's theme tune, as I'd hoped, so I'm still searching for that - and lots of other answers.
Hope this helps and doens't lead you (and I) down the wrong path!