Propose it should be turned in and that's why (6)
I believe the answer is:
motive
'why' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'propose it should be turned in' is the wordplay.
'propose' becomes 'move' (moving is a kind of proposing).
'should be turned' says the letters should be written backwards.
'in' indicates putting letters inside.
'it' reversed gives 'ti'.
'move' going around 'ti' is 'MOTIVE'.
'and that's' is the link.
Can you help me to learn more?
(Other definitions for motive that I've seen before include "Reason for doing something, perhaps a crime" , "Underlying intention" , "Reason (for acting)" , "Reason for committing a crime" , "Purpose in doing something" .)