Not fully trusting (5)
I believe the answer is:
leery
'not fully trusting' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for leery that I've seen before include "Suspiciously cautious" , "Sly, cunning" , "In slang, wary or suspicious - reely!" , "Knowing" , "Shy" .)