Successful student King George left out of praise (8)
I believe the answer is:
graduate
'successful student' is the definition.
(eg student graduating from university)
'king george left out of praise' is the wordplay.
'king george' becomes 'gr' (Georgius Rex).
'left' becomes 'l' (common abbreviation).
'out of' is a deletion indicator.
'praise' becomes 'adulate' (adulation is praise).
'adulate' with 'l' taken out is 'aduate'.
'gr'+'aduate'='GRADUATE'
(Other definitions for graduate that I've seen before include "Holder of university degree" , "Gain degree" , "University alumnus" , "Scholar" , "Recipient of degree" .)