Astonished second in command went to capture Marshal (4-4)
I believe the answer is:
open-eyed
'astonished' is the definition.
(I've seen this before)
'second in command went to capture marshal' is the wordplay.
'second in command' becomes 'o' (o is the second letter in command).
'went' becomes 'peed' (both can mean urinated).
'to capture' means one lot of letters goes inside another.
'marshal' becomes 'ney' (I've seen this in other clues).
'peed' going around 'ney' is 'peneyed'.
'o'+'peneyed'='OPEN-EYED'
(Other definitions for open-eyed that I've seen before include "Astonished; fully aware" , "Astonished - watchful" , "Surprised - aware" , "Forming an encrusted mass" .)