Arbitrary assertion (5)
'arbitrary assertion' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for say-so that I've seen before include "Authority - rumour" , "Authority to make a decision" , "Authorisation" , "Go-ahead" , "Authority to make or allow something to happen" .)
(Another definition for sayso that I've seen is " rumour".)