Mythical many-eyed giant (5)
I believe the answer is:
argus
'mythical many-eyed giant' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for argus that I've seen before include "Many-eyed mythical monster" , "Alert watchman" , "Small butterfly" , "Legendary Guardian" , "In Greek myth, a many-eyed monster slain by Hermes" .)