Donkey is on time to help (6)
I believe the answer is:
assist
'to help' is the definition.
(to assist someone is to help them)
'donkey is on time' is the wordplay.
'donkey' becomes 'ass' (ass is a term for a donkey).
'on' is a charade indicator (letters next to each other).
'time' becomes 't' (abbreviation).
'ass'+'is'+'t'='ASSIST'
(Other definitions for assist that I've seen before include "Work with N help" , "Work as a subordinate" , "Aid, abet" , "Understand by instinct" , "Help member" .)