it's too long to post here, but it's on AO3!

"“When did you start working here?” Clint asked, somewhere between the aviary and the puppy kennel. Before she could answer, he continued. “You weren’t there when I picked up Lucky, right? We were in and out of the Clinic a bunch, and,” he paused to give her a teasing grin, “I woulda remembered you, Red.”

Natasha resisted the urge to play with the ends of her ponytail at the reference to her hair. Contrary to the way that phrase normally sounded, the way he said it didn’t sound predatory or creepy - just curious. It was just a fact to him, and the nickname fell from his lips in such a way that it sounded familiar and friendly.

Clint Barton was a strange man."

OR

the Vet AU that turned into a 3k fic

http://archiveofourown.org/works/11855181