Think out the motive (6)
I believe the answer is:
reason
This clue is a double definition.
'think' is the first definition.
(I know that reason is a more specific form of the action the think)
'the motive' is the second definition.
(reason is a kind of motive)
'out' acts as a link.
(Other definitions for reason that I've seen before include "Motive; argue" , "Purpose" , "Logic and rationality" , "Justification or motive" , "Motivation or cause" .)