If really good, there will be none left (4,5)
I believe the answer is:
just right
'none left' is the definition.
Both the definition and answer are adverbs. Maybe there's a link between them I don't understand?
'if really good' is the wordplay.
I cannot quite understand how this works, but
'good' could be 'right' (synonyms) and 'right' is located in the answer.
The remaining letters 'just' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'there will be' is the link.
Can you help me to learn more?
(Other definitions for just right that I've seen before include "Exactly what was required" , "Perfect" .)