Not hidden, open (5)
I believe the answer is:
overt
I believe this clue is a double definition.
'not hidden' is the first definition.
(I've seen this before)
'open' is the second definition.
(synonyms)
(Other definitions for overt that I've seen before include "Open, up front" , "totally visible" , "Not concealed - public" , "Done or shown openly" , "This is not hard to see" .)