Lively youth: fine young fellow, on reflection (5)
I believe the answer is:
bucko
'lively youth' is the definition.
(bucko can mean a young man)
'fine young fellow on reflection' is the wordplay.
'fine' becomes 'OK' (synonyms).
'young fellow' becomes 'cub'.
'on reflection' says the letters should be written backwards.
'ok'+'cub'='okcub'
'okcub' in reverse letter order is 'BUCKO'.
(Other definitions for bucko that I've seen before include "One swaggering" , "Fellow swaggering" , "Young lad" , "Lively young Irish fellow" , "Young Irishman" .)