Help donkey first (6)
I believe the answer is:
assist
'help' is the definition.
(to assist someone is to help them)
'donkey first' is the wordplay.
'donkey' becomes 'ass' (ass is a term for a donkey).
'first' becomes 'ist' (ist resembles 1st).
'ass'+'ist'='ASSIST'
(Other definitions for assist that I've seen before include "Pitch in" , "Be helpful to" , "benefit" , "providing help?" , "Expedite" .)