Boy carrying short axe for German once (5)
I believe the answer is:
saxon
'german once' is the definition.
(the Saxons were a historical German people)
'boy carrying short axe' is the wordplay.
'boy' becomes 'son' (synonyms).
'carrying' is an insertion indicator.
'short axe' becomes 'ax' (synonyms; 'short' indicates an abbreviation).
'son' placed around 'ax' is 'SAXON'.
'for' is the link.
(Other definitions for saxon that I've seen before include "dating from the first millennium?" , "Member of old Germanic people" , "Germanic tribesman" , "Member of Germanic tribe settled in southern England after the Romans" , "old English style" .)