Expecting to be quiet as a ruler (8)
I believe the answer is:
pregnant
'expecting' is the definition.
(expecting can mean expecting a child)
'quiet as a ruler' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'as a ruler' becomes 'regnant' (eg a queen regnant reigns in her own right).
'p'+'regnant'='PREGNANT'
'to be' is the link.
(Other definitions for pregnant that I've seen before include "full of implications" , "Gravid" , "Expecting a baby" , "With child, enceinte" , "Carrying offspring in the womb" .)