Beast of burden arrived, then left (5)
I believe the answer is:
camel
'beast of burden' is the definition.
(thesaurus)
'arrived then left' is the wordplay.
'arrived' becomes 'came' (coming is a kind of arriving).
'then' says to put letters next to each other.
'left' becomes 'l' (common abbreviation).
'came'+'l'='CAMEL'
(Other definitions for camel that I've seen before include "Dromedary, perhaps" , "Light yellowish-brown colour - ruminant" , "A ship, some say" , "African transport" , "Beastly type" .)