German boy is happy (4)
I believe the answer is:
glad
'happy' is the definition.
(synonyms)
'german boy' is the wordplay.
'german' becomes 'g' (abbreviation).
'boy' becomes 'lad' (informal term for boy).
'g'+'lad'='GLAD'
'is' is the 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?" .)