Something illuminating lid? (3-6)
I believe the answer is:
eye-opener
This clue is a double definition.
'something illuminating' is the first definition.
(something which enlightens or illuminates)
'lid?' is the second definition.
(an eyelid)
(Other definitions for eye-opener that I've seen before include "Something startling" , "Circumstances that brings surprise and enlightenment" , "Surprising fact - like wake up call" , "Fact that brings great surprise - wakes you up" , "Surprising fact or circumstance of ocular origin" .)