Reversed major error bringing in monarch's rubbish (9)
I believe the answer is:
gibberish
'rubbish' is the definition.
(I've seen this before)
'reversed major error bringing in monarch's' is the wordplay.
'reversed' is a reversal indicator.
'major' becomes 'big' (I've seen this in other clues).
'error' becomes 'bish' (term for a blunder or mistake).
'bringing in' means one lot of letters goes inside another.
'monarch' becomes 'ER' (abbreviation for Elizabeth Regina, British Queen).
'big' reversed gives 'gib'.
'bish' going around 'er' is 'berish'.
'gib'+'berish'='GIBBERISH'
(Other definitions for gibberish that I've seen before include "Drivel" , "moonshine" , "Meaningless talk" , "Meaningless speech" , "rhubarb" .)