spiny: Fruition (or The Education of Arthur)
http://spiny.livejournal.com/4665.html

Arthur asked Merlin to look after a newbie knight, and promptly got jealous when Merlin did just that

Arthur had several cutting things to say to this, but Wynston was still watching—nay, leering—at Merlin and that took precedence. Presumptuous knights were not permitted to go laying hands on any of Arthur's things. He was the crown prince! Heir to the throne! And a righteously noble protector of the weak and insolent, which clearly described Merlin. Arthur knew his duty.

Aww Arthur gets jealous...

It was really all Sir Wynston's fault.

Arthur's jealous of Sir Wynston.

by spiny (NC-17) Arthur worries that a knight has designs on Merlin.

When Arthur asks Merlin to spy on Sir Wynston, it starts a chain of events that force Arthur to finally come to terms with what he feels for Merlin.