Toil with animal — one that could perform arduous tasks (9)
I believe the answer is:
workhorse
'one that could perform arduous tasks' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'toil with animal' is the wordplay.
'toil' becomes 'work' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'animal' becomes 'horse' (I've seen this before).
'work'+'horse'='WORKHORSE'
Can you help me to learn more?
(Other definitions for workhorse that I've seen before include "Reliable toiler" , "tireless toiler" , "Person (or machine) dependably slogging away" , "Punch maybe" , "giving regular service" .)