Born yesterday (5)
I believe the answer is:
naive
'born yesterday' 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 "Simple in outlook" , "Foolish, inexperienced" , "Unaffected or guileless" , "Rather too simple and gullible" , "Ingenuous, easily deceived" .)