Average American soldier (4)
I believe the answer is:
para
'soldier' is the definition.
(short for paratrouper)
'average american' is the wordplay.
'average' becomes 'par' (average score in golf).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'par'+'a'='PARA'
(Other definitions for para that I've seen before include "Airdropped soldier" , "High-jumper" , "Soldier dropping to earth" , "Air-drop soldier" , "Brazilian state" .)