Enormous American soldier acting badly (8)
I believe the answer is:
gigantic
'enormous' is the definition.
('gigantic' can be a synonym of 'enormous')
'american soldier acting badly' is the wordplay.
'american soldier' becomes 'gi'.
'badly' indicates an anagram.
'acting' with letters rearranged gives 'gantic'.
'gi'+'gantic'='GIGANTIC'
(Other definitions for gigantic that I've seen before include "monster" , "Sizeable" , "Extremely large" , "walloping" , "Hugh,colossal" .)