Not immediately obvious (6)
I believe the answer is:
subtle
'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" .)