Reason it came back in turn (6)
I believe the answer is:
motive
'reason' is the definition.
(I've seen this before)
'it came back in turn' is the wordplay.
'came back' is a reversal indicator.
'in' is an insertion indicator.
'turn' becomes 'move' (move is a kind of turn**).
'it' reversed gives 'ti'.
'ti' inserted into 'move' is 'MOTIVE'.
(Other definitions for motive that I've seen before include "What induces one to take a certain action" , "Underlying intention" , "Purpose in doing something" , "Incitement to do something" , "Underlying purpose" .)