Cardinal rule for a speaker (9)
I believe the answer is:
principal
'cardinal' is the definition.
(I've seen this before)
'rule for a speaker' is the wordplay.
'rule' becomes 'principle' (principle is a kind of rule).
'for a speaker' indicates a 'sounds like' (homophone) clue (the words would sound the same when spoken).
'principle' sounds like 'PRINCIPAL'.
(Other definitions for principal that I've seen before include "Chief, main" , "Headmaster" , "Chief - teacher?" , "top man" , "Cardinal" .)