Fully alert (4-4)
(Other definitions for open-eyed that I've seen before include "Forming an encrusted mass" , "Astonished - watchful" , "Astonished; fully aware" , "Surprised - aware" .)
'fully alert' is the definition.
(I've seen this before)
This is all the clue.
(Another definition for wide-awake that I've seen is " Fully alert".)