I mentioned first item's unexpected revelation (3,6)
I believe the answer is:
eye opener
'unexpected revelation' is the definition.
(I've seen this before)
'i mentioned first item's' is the wordplay.
I cannot really understand how this works, but
'i' could be 'eye' (I've seen this in other clues) and 'eye' is located in the answer.
'first' could be 'opener' (I've seen this before) and 'opener' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for eye opener that I've seen before include "it might be revealing" , "one lifts the lid?" , "causing surprise" , "Revealing thing" , "big surprise!" .)