Beast of burden given first aid (6)
I believe the answer is:
assist
'aid' is the definition.
(I know that aid can be written as assist)
'beast of burden given first' is the wordplay.
'beast of burden' becomes 'ass' (animal which carries a load).
'given' says to put letters next to each other.
'first' becomes 'ist' (ist resembles 1st).
'ass'+'ist'='ASSIST'
(Other definitions for assist that I've seen before include "Be helpful to" , "Give aid" , "Help member" , "Understand by instinct" , "providing help?" .)