Simple or guileless (5)
I believe the answer is:
naive
I believe this is a double definition.
'simple' is the first definition.
(similar in meaning)
'guileless' is the second definition.
(I've seen this in another clue)
(Other definitions for naive that I've seen before include "Ingenuous, easily deceived" , "Jejune" , "Natural, unaffected" , "Artless; simplistic" , "Credulous, unworldly" .)