Possibly sit on a donkey to help (6)
I believe the answer is:
assist
'to help' is the definition.
(to assist someone is to help them)
'possibly sit on a donkey' is the wordplay.
'possibly' is an anagram indicator.
'on' says to put letters next to each other.
'a donkey' becomes 'ass' (ass is a term for a donkey).
'sit' with letters rearranged gives 'ist'.
'ist' after 'ass' is 'ASSIST'.
(Other definitions for assist that I've seen before include "Give a hand to" , "Give aid" , "Help, lend a hand" , "Understand by instinct" , "Be useful to" .)