Fruit fruitless fruits (6)
I believe the answer is:
grapes
'fruits' is the definition.
(grape is a kind of fruit)
'fruit fruitless' is the wordplay.
'fruit' becomes 'grape' (grape is a kind of fruit).
'fruitless' becomes 's' (I can't explain this - if you can you should give a lot more credence to this answer).
'grape'+'s'='GRAPES'
Can you help me to learn more?
(Other definitions for grapes that I've seen before include "Vine fruit" , "Vine fruits" , "'Green, black or purple clusters (6)'" , "Source of wine" , "Juicy fruits" .)