Happy, light-hearted youth (4)
I believe the answer is:
glad
'happy' is the definition.
(I know that happy can be written as glad)
'light-hearted youth' is the wordplay.
'hearted' indicates the central letters.
'youth' becomes 'lad' (lad means a young man).
The centre of 'light' is 'g'.
'g'+'lad'='GLAD'
(Other definitions for glad that I've seen before include "bright?" , "Happy, cheerful" , "More than willing" , "Elated" , "Pleased, delighted" .)