Why shift it back inside? (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.
'shift it back inside?' is the wordplay.
'shift' becomes 'move' (I've seen this before).
'back' is a reversal indicator.
'inside?' indicates putting letters inside.
'it' backwards is 'ti'.
'move' enclosing 'ti' is 'MOTIVE'.
Can you help me to learn more?
(Other definitions for motive that I've seen before include "Inspiration" , "Incitement to do something" , "Reason for doing something, say murder" , "Underlying intention" , "Ground" .)