Article, current, on unknown Antipodean soldier (5)
I believe the answer is:
anzac
'antipodean soldier' is the definition.
(Australian and New Zealand Army Corps soldier)
'article current on unknown' is the wordplay.
'article' becomes 'an' (indefinite article).
'current' becomes 'ac' (alternating current).
'on' means one lot of letters go next to another.
'unknown' becomes 'z' (algebra).
'ac' after 'z' is 'zac'.
'an'+'zac'='ANZAC'
(Other definitions for anzac that I've seen before include "Planet rotates about it" , "Old warrior" , "World War One Antipodean soldier" , "World War One Australian or NZ soldier" , "digger" .)