Boy (half German) accepts summons to be a musician (10)
I believe the answer is:
songwriter
'a musician' is the definition.
(songwriter is a kind of musician)
'boy half german accepts summons' is the wordplay.
'boy' becomes 'son' (synonyms).
'half' indicates taking half.
'accepts' means one lot of letters goes inside another.
'summons' becomes 'writ' (I've seen this before).
'german' halved is 'ger'.
'ger' enclosing 'writ' is 'gwriter'.
'son'+'gwriter'='SONGWRITER'
'to be' is the link.
(Other definitions for songwriter that I've seen before include "Producer of music/lyrics" , "One who works with numbers" , "Composer" , "Berlin, perhaps" .)