A beast of burden is the first to help (6)
I believe the answer is:
assist
'help' is the definition.
(to assist someone is to help them)
'a beast of burden is the first' is the wordplay.
'a beast of burden' becomes 'ass' (animal which carries a load).
'the first' becomes 't' (1st letter of 'the').
'ass'+'is'+'t'='ASSIST'
'to' is the link.
(Other definitions for assist that I've seen before include "Provide aid, help out" , "Give a hand to" , "Support" , "Understand by instinct" , "Give help" .)