Thinking it over in advance? (6)
I believe the answer is:
motive
'thinking' is the definition.
(both can mean someone's reason for doing something)
'it over in advance?' is the wordplay.
'over' indicates anagramming the letters.
'in' indicates putting letters inside.
'advance?' becomes 'move' (advance is a kind of move**).
'it' with letters rearranged gives 'ti'.
'ti' placed into 'move' is 'MOTIVE'.
(Other definitions for motive that I've seen before include "Purpose in doing something" , "Reason behind an action" , "Ground" , "Reason (for acting)" , "Rationale, purpose" .)