Wonder what a lid could be? (3-6)
I believe the answer is:
eye-opener
'wonder' is the definition.
(I've seen this before)
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 that brings great surprise, dilating optics" , "Fact that surprises or enlightens in optical way" , "Unexpected revelation" , "Surprising fact - like wake up call" .)