What nanny may say, with maiden in royal address (4)
I believe the answer is:
maam
'royal address' is the definition.
(how to address the Queen)
'what nanny may say with maiden' is the wordplay.
'what nanny may say' becomes 'maa' (the noise a goat makes).
'with' is a charade indicator (letters next to each other).
'maiden' becomes 'm' (cricket abbreviation).
'maa'+'m'='MAAM'
'in' acts as a link.
(Other definitions for maam that I've seen before include "Respectful address to the Queen" , "HRH" , "Form of address to female royalty" , "Polite address to Queen" , "Formal address to a lady" .)