Undecided, to be frank (4)
I believe the answer is:
open
I believe this clue is a double definition.
'undecided' is the first definition.
('open' can be a synonym of 'undecided')
'frank' is the second definition.
('open' can be a synonym of 'frank')
'to be' is the link.
(Other definitions for open that I've seen before include "Agape" , "Frank; not closed" , "Traditional room heater" , "Liberal" , "Not secret or hidden" .)