E cryptically? It might be a surprise (3-6)
I believe the answer is:
eye-opener
'a surprise' is the definition.
(eye opener is a kind of surprise)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for eye-opener that I've seen before include "Startling revelation" , "Fact or circumstance that brings enlightenment and surprise" , "This surprises greatly - makes you look" , "Surprising fact - like wake up call" , "Fact that surprises or enlightens in optical way" .)