Tony had made something for Clint, possibly arrowheads, and was giving them to him while talking with Steve & Natasha. They ask/tease why he doesn't give Natasha any presents and he replies that he doesn't make/give people things they can use to kill him. Natasha tells him that she can use anything he gives her to kill him, to which he fires back 'exactly!', although they're both being good-natured about it. Later on I think he upgrades her Widow's bites for her?
It was on Ao3, and possibly part of a long fic.
9/9/15 edit: Found it, Honey, I can see the stars