Cheerful boy, by George (to begin with) (4)
I believe the answer is:
glad
'cheerful' is the definition.
(similar in meaning)
'boy by george to begin with' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'by' is a charade indicator (letters next to each other).
'to begin with' suggests taking the first letters.
The initial letter of 'george' is 'g'.
'lad' after 'g' is 'GLAD'.
(Other definitions for glad that I've seen before include "These rags are dressy clothes - happy?" , "Made up" , "More than willing" , "Elated" , "Informal flower < -girl" .)