Rambling diatribe by the Queen! (6)
I believe the answer is:
errant
'rambling' is the definition.
(thesaurus)
'diatribe by the queen' is the wordplay.
'diatribe' becomes 'rant' (I've seen this in other clues).
'by' means one lot of letters go next to another.
'the queen' becomes 'er' (Elizabeth Regina).
'rant' after 'er' is 'ERRANT'.
(Other definitions for errant that I've seen before include "Offending" , "Wrong" , "Misbehaving" , "Travelling about or going astray" , "looking for adventure" .)