Not blatant (6)
I believe the answer is:
'not blatant' 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" , "Slight and difficult to detect, like some humour" , "Slight and difficult to analyse amid bustle" , "Able to make fine distinctions" , "Understated - discreetly clever" .)