Cunning description of the National Gallery? (6)
I believe the answer is:
artful
'cunning' is the definition.
('artful' can be similar in meaning to 'cunning')
I can't explain the rest of the clue.
(Other definitions for artful that I've seen before include "Oliver Twist's pal was an . . . . . . dodger" , "Giving a false appearance of frankness" , "Clever like Dickens's Dodger" , "Unnatural - cunning" , "showing craft?" .)