Guileless (5)
I believe the answer is:
naive
'guileless' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for naive that I've seen before include "Unaffected; credulous" , "Ingenuous, easily deceived" , "Rather too innocent and trusting" , "Showing lack of maturity or judgement" , "Wet behind ears" .)