Boy with German tune (4)
I believe the answer is:
song
'tune' is the definition.
(thesaurus)
'boy with german' is the wordplay.
'boy' becomes 'son' ('son' can be a synonym of 'boy'**).
'with' says to put letters next to each other.
'german' becomes 'g' (abbreviation).
'son'+'g'='SONG'
(Other definitions for song that I've seen before include "Carol, for example" , "Vocal number" , "Musical composition; air" , "Words set to music" , "You work best when on this" .)