Observer sees insect reaching vertical surface (3,2,3,4)
I believe the answer is:
fly on the wall
'observer' is the definition.
(a fly on the wall secretly observes things)
'insect reaching vertical surface' is the wordplay.
I cannot quite understand how this works, but
'insect' could be 'fly' (fly is a kind of insect) and 'fly' is located in the answer.
This explanation may well be incorrect...
'sees' is the link.
Can you help me to learn more?
(Other definitions for fly on the wall that I've seen before include "Unobserved observer" , "Unnoticed observer" , "inconspicuous observer?" , "Bug in room" , "for secret observation?" .)