Unseen observer (3,2,3,4)
I believe the answer is:
fly on the wall
'unseen observer' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for fly on the wall that I've seen before include "Unobserved observer" , "Unseen observer" , "Unnoticed observer" , "An invisible observer of an event" , "inconspicuous observer?" .)