... certain GI sniper holding up badge (8)
I believe the answer is:
insignia
'badge' is the definition.
(I know that insignia is a type of badge)
'certain gi sniper holding up' is the wordplay.
'holding' indicates the answer is hidden within the clue.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'aingisni' is hidden within 'certain gi sniper'.
'aingisni' in reverse letter order is 'INSIGNIA'.
(Other definitions for insignia that I've seen before include "marks" , "Emblems of rank" , "military mark" , "Badge worn to show official position" , "Stars and stripes, say" .)