Wonderful hitting tennis ball hard (8)
I believe the answer is:
smashing
'wonderful' is the definition.
(I've seen this before)
'hitting tennis ball hard' is the wordplay.
I cannot quite understand how this works, but
'hitting' could be 'smash' (smash is a kind of hitting) and 'smash' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for smashing that I've seen before include "Such destructive behaviour" , "use for plates at a Greek wedding?" , "Committing act of vandalism" , "Great" , "ace" .)