Rambling diatribe in support of the queen (6)
I believe the answer is:
errant
'rambling' is the definition.
(thesaurus)
'diatribe in support of the queen' is the wordplay.
'diatribe' becomes 'rant' (I've seen this in other clues).
'in support of' says to put letters next to each other (I've seen this in other clues).
'the queen' becomes 'er' (Elizabeth Regina).
'rant' after 'er' is 'ERRANT'.
(Other definitions for errant that I've seen before include "heretical?" , "looking for adventure" , "so some knights are called" , "Quixotic" , "Straying from the right course" .)