Happy German boy (4)
I believe the answer is:
glad
'happy' is the definition.
(I know that happy can be written as glad)
'german boy' is the wordplay.
'german' becomes 'g' (abbreviation).
'boy' becomes 'lad' (informal term for boy).
'g'+'lad'='GLAD'
(Other definitions for glad that I've seen before include "happiness" , "Little flowering plant" , "Feeling pleasure" , "Happy, cheerful" , "More than willing" .)