Unexpected revelation (3-6)
'unexpected revelation' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for eye-opener that I've seen before include "This surprises greatly - makes you look" , "A startling enlightenment!" , "Fact that brings great surprise - wakes you up" , "Startling thing" , "Surprising fact - like wake up call" .)
(Other definitions for eyeopener that I've seen before include "piece of information that's surprising" , "revelation" , "For Uncle Sam, perhaps hair of the dog" .)