Final authority (3-2)
'final authority' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for say-so that I've seen before include "Authorisation" , "Authority to make or allow something to happen" , "Authority - rumour" , "Authority to make a decision" , "Go-ahead" .)
(Another definition for sayso that I've seen is " rumour".)