Open secret, but not to begin with (5)
I believe the answer is:
overt
'open' is the definition.
(associated in meaning)
'secret but not to begin with' is the wordplay.
'secret but' becomes 'vert' (I can't justify this - if you can you should believe this answer much more).
'not' becomes 'o' (I've seen this in other clues).
'to begin with' says to put letters next to each other.
'vert' after 'o' is 'OVERT'.
Can you help me to learn more?
(Other definitions for overt that I've seen before include "Open and undisguised" , "Not locked away" , "Open, not 1 across" , "totally visible" , "Not concealed - public" .)