A fool it's wrong to help (6)
I believe the answer is:
assist
'to help' is the definition.
(to assist someone is to help them)
'a fool it's wrong' is the wordplay.
'a fool' becomes 'ass' (both can mean a stupid person).
'wrong' indicates anagramming the letters.
'its' is an anagram of 'ist'.
'ass'+'ist'='ASSIST'
(Other definitions for assist that I've seen before include "Be helpful to" , "provide backing" , "Understand by instinct" , "benefit" , "Be useful to" .)