The Sunday times crossword is no longer any fun. Take 19d. Shoes quickly put on in boring flight. (A.) Step ins. Step ins are US speak for womens' panties, shoes are slip ons. Difficult to see the link to boring flight?
Boring in the sense of drilling in to. "in" boring (into) "flight" (steps) gives step(in)s. I'm in the US and have never heard step-ins refer to panties, so can't comment on that.