Happy, light-hearted youth (4)
I believe the answer is:
glad
'happy' is the definition.
('glad' can be a synonym of 'happy')
'light-hearted youth' is the wordplay.
'hearted' means to look at the middle letters.
'youth' becomes 'lad' (I've seen this before).
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" .)