Nutritious fruit mostly very good? Excellent (4)
I believe the answer is:
acai
'excellent' is the definition.
I know nothing about this answer so I can't judge whether this works.
'nutritious fruit mostly very good?' is the wordplay.
'nutritious fruit' becomes 'aca' (I can't explain this - if you can you should believe this answer much more).
'mostly' means to remove the first letter (a).
'very good?' becomes 'AI' (resembles 'Aa', old-fashioned term for excellent).
'ai' with its initial letter removed is 'i'.
'aca'+'i'='ACAI'
Can you help me to learn more?
(Other definitions for acai that I've seen before include "South American food product" , "something nutritional to eat" , "fruit which is nutritious" , "South American palm tree" .)