Authority is silent? (3-2)
I believe the answer is:
say-so
'authority' is the definition.
(I know that authority can be written as say-so)
I don't understand how the remainder of the clue works.
(Other definitions for say-so that I've seen before include "Final approval" , "Authority to make or allow something to happen" , "Authority to make a decision" , "Mere assertion (colloq.)" , "Go-ahead" .)