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 can't see how one could define the other.
'shift it back inside?' is the wordplay.
'shift' becomes 'move' (I've seen this before).
'back' shows that the letters should be reversed in order.
'inside?' means one lot of letters goes inside another.
'it' back-to-front is 'ti'.
'move' going around '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" .)