Mathi, for 15 D, the first three letters in the answer = first two words in the clue, the next three letters = the third word and the last two = the next three words.
Mathi, for 24 A - it doesn't! The 'outfit' is the four letters in the middle of the answer, the other two come from taking ('going') the 'old girlfriend' out of 'sexy'.
Yes, the setters enjoy obscure definitions like rat=grass, etc! I wasn’t aware this. Though we can get the answers from the crossers, I find the parsing quite difficult sometimes.