Very silly person with unnatural intelligence (5)
I believe the answer is:
assai
'very' is the definition.
(I've seen this before)
'silly person with unnatural intelligence' is the wordplay.
'silly person' becomes 'ass' (I've seen this in another clue).
'with unnatural intelligence' becomes 'ai' (I can't explain this - if you can you should give a lot more credence to this answer).
'ass'+'ai'='ASSAI'
Can you help me to learn more?
(Other definitions for assai that I've seen before include "Palm tree" , "Very (mus.)" , "Produced offspring" , "Musician's very" , "Very (in musical score)" .)