Able to make fine distinctions (6)
I believe the answer is:
subtle
'able to make fine distinctions' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for subtle that I've seen before include "Understated; devious" , "Capable of making fine distinctions" , "Difficult to analyse; understated" , "Not blatant" , "Able to make fine distinctions" .)