Betting fellow has done - very good! (8)
I believe the answer is:
splendid
'very good' is the definition.
(thesaurus)
'betting fellow has done' is the wordplay.
I cannot quite understand how this works, but
'betting' could be 'sp' and 'sp' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for splendid that I've seen before include "'Excellent, glorious (8)'" , "Very good, terrific" , "Grand" , "Excellent, magnificent" , "Characterised by grandeur" .)