German boy is happy (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'
'is' acts as a link.
(Other definitions for glad that I've seen before include "Not sorry" , "Happy, cheerful" , "Feeling happy" , "Feeling pleasure" , "These rags are dressy clothes - happy?" .)