Not too sure (5)
I believe the answer is:
leery
'not too sure' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for leery that I've seen before include "Cunning - wary" , "Suspiciously cautious" , "Cautious due to suspicions" , "Sly, cunning" , "Shy" .)