Simple and guileless (5)
I believe the answer is:
naive
'simple and guileless' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for naive that I've seen before include "Innocent or not worldly enough" , "Ingenous, easy to gull" , "Unsophisticated and gullible" , "Unenlightened" , "Lacking worldly experience" .)