Fool is last to accept help (6)
I believe the answer is:
assist
'help' is the definition.
(assisting is a kind of helping)
'fool is last to accept' is the wordplay.
'fool' becomes 'ass' (ass is a kind of fool**).
'last to' says to take the final letters.
The last letter of 'accept' is 't'.
'ass'+'is'+'t'='ASSIST'
(Other definitions for assist that I've seen before include "Provide aid, help out" , "providing help?" , "Be useful to" , "provide backing" , "Give aid" .)