Act the worrywart

FRET

Think of what a musician might do to a guitar string—or how you might react when a small problem keeps nagging at you.

"Act the worrywart" is a playful way to say someone is behaving anxiously or overreacting to stress. The word "act" hints that you're looking for a verb, and "worrywart" is a casual term for someone who frets constantly—so the answer is simply "FRET," meaning to worry unnecessarily. The clue cleverly combines the idea of performing ("act") with the trait of being a worrywart to point straight to the word.