Tirade supports former monarch going off the rails (6)
I believe the answer is:
errant
'going off the rails' is the definition.
I can't judge whether this defines the answer.
'tirade supports former monarch' is the wordplay.
'tirade' becomes 'rant' (both can mean an angry outburst).
'supports' is a charade indicator (letters next to each other) (bottom letters provide a support for the top ones).
'former monarch' becomes 'ER' (Elizabeth Regina, ie Queen Elizabeth).
'rant' put after 'er' is 'ERRANT'.
Can you help me to learn more?
(Other definitions for errant that I've seen before include "Nomadic" , "Straying; knight" , "Root vegetable" , "Offending" , "Like many a knight" .)