Gone in the end, slug with eyes on stalks (4)
I believe the answer is:
agog
'with eyes on stalks' is the definition.
(both can mean astonished)
'gone in the end slug' is the wordplay.
'gone' becomes 'ago' (eg 'two weeks gone' could mean 'two weeks ago').
'in the end' says to take the final letters.
The final letter of 'slug' is 'g'.
'ago'+'g'='AGOG'
(Other definitions for agog that I've seen before include "Enthusiastic" , "Eager and wide-eyed" , "Excitedly eager" , "On tenterhooks" , "Receptive - wide-mouthed" .)