Not immediately obvious (6)
I believe the answer is:
'not immediately obvious' is the definition.
(subtleties are not immediately obvious)
This is all the clue.
(Other definitions for subtle that I've seen before include "Bustle in crafty, angenious way" , "Very fine, difficult to perceive or appreciate" , "Not blatant" , "Ingenious, difficult to analyse" , "Able to make fine distinctions" .)