An acoustic device for discovering the opinion of the directors! (8,5)
I believe the answer is:
sounding board
'an acoustic device for' is the definition.
The definition suggests a singular noun which matches the answer.
'the opinion of the directors' is the wordplay.
'the opinion of' becomes 'sounding' (I can't explain this - if you can you should give a lot more credence to this answer).
'the directors' becomes 'board' (board of directors).
'sounding'+'board'='SOUNDING-BOARD'
'discovering' acts as a link.
Can you help me to learn more?
(Other definitions for sounding board that I've seen before include "Means of testing opinion" , "means of testing idea" , "One whose opinion is sought" , "Recipient of experimental ideas perhaps" .)