Too trusting (5)
I believe the answer is:
'too trusting' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for naive that I've seen before include "Simple, innocent or unsophisticated" , "Artless, innocent" , "Gullible, too trusting" , "Ingenuous, easily deceived" , "Ingenuous and credulous" .)