Can't you tell if it's a dark one? (6)
I believe the answer is:
secret
'one?' is the definition.
Both the definition and answer are adjectives. Perhaps you can see a link between them that I don't see?
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for secret that I've seen before include "Esoteric" , "Private; classified" , "hidden from general view" , "One not to be divulged" , "avoiding exposure" .)