Think logically (6)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
reason
'think logically' is the definition.
(synonyms)
This is all the clue.
(Other definitions for reason that I've seen before include "Power of thought" , "Cause, justification" , "Sanity of senora" , "Motivation or cause" , "Rational motive" .)