Why you get some time in stir (6)
I believe the answer is:
motive
'why' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for motive that I've seen before include "Purpose - reason" , "Incitement to do something" , "Purpose in doing something" , "Reason for committing a crime" , "What induces one to take a certain action" .)