A fool’s one is fruitless (6)
I believe the answer is:
errand
'a fool's' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'one is fruitless' is the wordplay.
I cannot really see how this works, but
'one' could be 'an' (I've seen this in other clues) and 'an' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for errand that I've seen before include "Short journey to carry message for Renard" , "Mission (usually run!)" , "Mission (often run)" , "Job usually involving a short trip" , "Delivery/collection trip" .)