Beast of burden arrived on lake (5)
I believe the answer is:
camel
'beast of burden' is the definition.
(I've seen this before)
'arrived on lake' is the wordplay.
'arrived' becomes 'came' (come can mean to appear or arrive).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'lake' becomes 'L' (geographical abbreviation).
'came'+'l'='CAMEL'
(Other definitions for camel that I've seen before include "Saddle animal in desert regions" , "Humped beast" , "Dromedary, perhaps" , "Humped ungulate" , "Ungulate mammal" .)