A mythical giant with eyes on stalks (4)
I believe the answer is:
agog
'with eyes on stalks' is the definition.
(both can mean astonished)
'a mythical giant' is the wordplay.
'mythical giant' becomes 'gog' (folklore figure).
'a'+'gog'='AGOG'
(Other definitions for agog that I've seen before include "Very eager (to hear/see)" , "Receptive - wide-mouthed" , "Keen" , "Impatient" , "State of wide-eyed wonder" .)