Happy girl's first young man (4)
I believe the answer is:
glad
'happy' is the definition.
(synonyms)
'girl's first young man' is the wordplay.
'girl's first' becomes 'g' (1st letter of 'girl').
'young man' becomes 'lad' (lad can mean a boy or young man).
'g'+'lad'='GLAD'
(Other definitions for glad that I've seen before include "Happy, cheerful" , "bright?" , "Little flowering plant" , "Happy, pleased" , "Contented" .)