Overtime rates disheartened these beasts of burden (7)
I believe the answer is:
onagers
'beasts of burden' is the definition.
The definition and answer can be both animals as well as being plural nouns.
Perhaps they are linked in a way I don't understand?
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for onagers that I've seen before include "Asian wild asses" , "Asiatic wild asses" , "wild animals" , "Strange oranges for the wild asses" .)