Crossword Genius

Observer overlooked what another type of paper would catch? (1,3,2,3,4)

Ross

I believe the answer is:

a fly on the wall

'observer overlooked' is the definition.
(a fly on the wall is a hidden or unseen observer)

I don't understand the remainder of the clue.

(Other definitions for a fly on the wall that I've seen before include "What we'd often like to be" , "Enviable spying position" , "An unnoticed observer" .)

I've seen this clue in The Times.
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