Board's main political principle (5)
I believe the answer is:
plank
This clue is a double definition.
'board's' is the first definition.
('plank' can be a synonym of 'board')
'main political principle' is the second definition.
(Other definitions for plank that I've seen before include "Wooden board" , "Long piece of timber" , "Policy; board" , "flat piece of timber" , "Proverbially thick piece of wood" .)