Not so wonderful that you can't leave behind? (8)
I believe the answer is:
passable
'behind?' is the definition.
Both the definition and answer are adjectives. Perhaps they are linked in a way I don't understand?
'not so wonderful that you can't' is the wordplay.
I cannot really understand how this works, but
'so' could be 'as' (e.g. phrases like 'not as tall' and 'not so tall') and 'as' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'leave' is the link.
Can you help me to learn more?
(Other definitions for passable that I've seen before include "Merely satisfactory" , "Barely adequate" , "okay?" , "examinations should be!" , "OK" .)