Moving up levels in SHIELD comes with new requirements and responsibilities, as promotion does. Natasha reaches the level where she has to teach/train others. (She can't wait to be classed as a Specialist so she doesn't have to deal with this shit. Not that Specialists don't teach/train at all, but it's rare and on request and refuseable, not part of the job description with topics chosen by senior officers.)

In other words: as part of the undercover training course Natasha has to teach seduction and Clint signs up. (Being ine if the good guys means she's not allowed to kill him, right?)

(A SHEILD early-ish days fic)