Queen's tirade rambling (6)
I believe the answer is:
errant
'rambling' is the definition.
(I've seen this before)
'queen's tirade' is the wordplay.
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'tirade' becomes 'rant' (both can mean an angry outburst).
'er'+'rant'='ERRANT'
(Other definitions for errant that I've seen before include "Wrong" , "Misbehaving" , "Quixotic" , "Offending" , "Straying from the right course" .)