Happy girl's first young man (4)
I believe the answer is:
glad
'happy' is the definition.
(I know that happy can be written as glad)
'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" .)