Fighting the German keeper (6)
I believe the answer is:
warder
'keeper' is the definition.
(thesaurus)
'fighting the german' is the wordplay.
'fighting' becomes 'war'.
'the german' becomes 'der' ('the' in German).
'war'+'der'='WARDER'
(Other definitions for warder that I've seen before include "keeper" , "Gaoler" , "Jailer" , "One is in charge of prisoners" , "Prison officer" .)