I went through the Writer!Bucky Barnes AO3 tag but nothing jogged my brain on the summaries or titles unless I'm just particularly stupid this morning.
I remember he works with/for Natasha.
Situation that sticks in my head (an explanation in the fic) says she gave him some work once to just proofread and after a point he got fed up and grabbed a pen and started making comments in the margins, some of them pretty harsh and some funny. A lot like "Are you kidding me?" and stuff like that.
That's like the biggest part, Bucky is a sarcastic editor. Everything else escapes me.
I am 99% sure it is Steve/Bucky because that's a favorite of mine.