Reason for doing something (6)
I believe the answer is:
motive
'reason for doing something' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for motive that I've seen before include "Reason for doing something, perhaps a crime" , "Incitement to do something" , "Inspiration" , "Purpose in doing something" , "Reason (for acting)" .)