Naive, artless (9)
I believe the answer is:
ingenuous
I believe this is a double definition.
'naive' is the first definition.
(similar in meaning)
'artless' is the second definition.
(synonyms)
(Other definitions for ingenuous that I've seen before include "lacking sophistication" , "green" , "Naive" , "Frank" , "Innocent, unsuspecting" .)