Cardinal rule for the audience (9)
I believe the answer is:
principal
'cardinal' is the definition.
(I've seen this before)
'rule for the audience' is the wordplay.
'rule' becomes 'principle' (principle is a kind of rule).
'for the audience' indicates a 'sounds like' (homophone) clue (how an audience might hear it).
'principle' is a homophone of 'PRINCIPAL'.
(Other definitions for principal that I've seen before include "Chief, main" , "British civil servant" , "Cardinal" , "property" , "Key" .)