Ahead, having broken Djokovic? Fantastic! (6)
I believe the answer is:
superb
'fantastic' is the definition.
(I've seen this in another clue)
'ahead having broken djokovic?' is the wordplay.
'ahead' becomes 'up' (similar in meaning).
'having broken' means one lot of letters goes inside another.
'djokovic?' becomes 'serb' (I can't justify this - if you can you should give a lot more credence to this answer).
'up' put within 'serb' is 'SUPERB'.
Can you help me to learn more?
(Other definitions for superb that I've seen before include "Dazzling" , "choice" , "Wonderful, really great" , "Quite splendid, really excellent" , "'Excellent, splendid (6)'" .)