It communicates with the dead plank, after a double affirmative (5,5)
I believe the answer is:
ouija board
'it communicates with the dead' is the definition.
The definition suggests a singular noun which matches the answer.
'plank after a double affirmative' is the wordplay.
'plank' becomes 'board' (synonyms).
'after' means one lot of letters go next to another.
'a double affirmative' becomes 'ouija' (I am not sure about this - if you are sure you should believe this answer much more).
'board' after 'ouija' is 'OUIJA-BOARD'.
Can you help me to learn more?
(Other definitions for ouija board that I've seen before include "Medium's accessory" , "panel conveying messages" .)