Routine work that's laborious and cyclical in nature (9)
I believe the answer is:
'routine' is the definition.
'work that's laborious and cyclical in nature' is the wordplay.
I cannot quite understand how this works, but
'work' could be 'read' (read is a kind of work) and 'read' is located in the answer.
'nature' could be 'ill' (ill nature is a kind of nature) and 'ill' is present in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for treadmill that I've seen before include "hard labour" , "Daily grind" , "Dreary round" , "Drill team (anag) - routine drudgery" , "Wearisome routine" .)