A game soldier taking on American defence (8)
I believe the answer is:
apologia
'defence' is the definition.
(apologia is a kind of defence)
'a game soldier taking on american' is the wordplay.
'game' becomes 'polo' (polo is a kind of game).
'soldier' becomes 'GI' (term for US soldier).
'taking on' is a charade indicator (letters next to each other).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'a'+'polo'+'gi'+'a'='APOLOGIA'
(Other definitions for apologia that I've seen before include "Written defence or vindication" , "Written defence of one's conduct" , "Personal written defence" , "form of defence" , "sort of defence" .)