Reveal the true nature of (6)
I believe the answer is:
unmask
'reveal the true nature of' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for unmask that I've seen before include "Expose, remove disguise" , "Discover the identity of" , "Expose the true character of (a person)" , "Expose and denounce" , "Reveal true identity of" .)