After midnight the boy is happy (4)
I believe the answer is:
glad
'is happy' is the definition.
(synonyms)
'after midnight the boy' is the wordplay.
'after' means one lot of letters go next to another.
'midnight' becomes 'g' (the middle letter in 'night').
'the boy' becomes 'lad' (informal term for boy).
'g' put next to 'lad' is 'GLAD'.
(Other definitions for glad that I've seen before include "Little flowering plant" , "Feeling pleasure" , "bright?" , "Made up" , "Pleased, delighted" .)