Hidden observer (3,2,3,4)

I believe the answer is:

fly on the wall

(Other definitions for fly on the wall that I've seen before include "Bug in room" , "Unobserved observer" , "Unseen observer" , "Unobtrusive documentary" , "inconspicuous observer?" .)

I've seen this clue in The Mirror.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.