Game soldier entered as defence (8)
I believe the answer is:
apologia
'defence' is the definition.
(apologia is a kind of defence)
'game soldier entered as' is the wordplay.
'game' becomes 'polo' (polo is a kind of game).
'soldier' becomes 'GI' (GI can mean a US soldier).
'entered' means one lot of letters goes inside another.
'as' becomes 'AA' (two letter A's).
'polo'+'gi'='pologi'
'pologi' inserted into 'aa' is 'APOLOGIA'.
(Other definitions for apologia that I've seen before include "Formal defence of one's actions" , "Written defence or vindication" , "form of defence" , "Formal defence of belief or conduct" , "Personal written defence" .)