Feeding an assembly that's very loud (9)
I believe the answer is:
deafening
'very loud' is the definition.
(I've seen this before)
'feeding an assembly' is the wordplay.
'assembly' indicates anagramming the letters.
'feeding'+'an'='feedingan'
'feedingan' anagrammed gives 'DEAFENING'.
'that's' acts as a link.
(Other definitions for deafening that I've seen before include "Noisy" , "Very loud" .)