Not sure I really get 5A & 6A... unless it's like... "these clues are so commonly used and are now boring, thus, you put one letter in and fill the rest with Zs as if you've fallen asleep?"
For those who didn't solve the NYT and need an explanation, in both 5A and 6A, a "SLEEPER" car in the entry has been replaced with Z's - REO and KIA respectively.