Board’s strategy number eleven? (5)
I believe the answer is:
plank
'board's' is the definition.
('plank' can be a synonym of 'board')
'strategy number eleven?' is the wordplay.
'strategy' becomes 'plan' (I've seen this before).
'number eleven?' becomes 'k' (eleventh letter of the alphabet).
'plan'+'k'='PLANK'
(Other definitions for plank that I've seen before include "item in political party's programme" , "Long piece of wood" , "Long flat timber" , "Length of sawn timber" , "Policy; board" .)