Board’s strategy number eleven? (5)
I believe the answer is:
Here is my best explanation:
'board's' is the definition.
('plank' can be a synonym of 'board')
'strategy number eleven?' is the subsidiary indication.
'strategy' becomes 'plan' (I've seen this before).
'number eleven?' becomes 'k' (eleventh letter of the alphabet).
(Other definitions for plank that I've seen before include "Building timber", "Wooden board", "length", "Length of sawn timber", "Long flat timber", "Length of wood", "Long piece of timber", "item in political party's programme", "Long piece of wood" and "flat piece of timber".)