Seemed unable to speak, briefly, after first three ciders over there, but then made a solemn pronouncement (6)
I believe the answer is:
dictum
'pronouncement' is the definition.
(I know that pronouncement can be written as dictum)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for dictum that I've seen before include "Short statement of a general principle" , "Formal statement - maxim" , "Popular maxim" , "Authoritative declaration" , "Fiat" .)