Her Majesty not disguising herself in rubber (6)
I believe the answer is:
eraser
'rubber' is the definition.
(rubber can mean an eraser)
'her majesty not disguising herself' is the wordplay.
I cannot quite see how this works, but
'her majesty' could be 'er' (Elizabeth Regina) and 'er' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'in' acts as a link.
(Other definitions for eraser that I've seen before include "Required, should you make a mistake" , "Item used for rubbing out" , "Rubber for removing marks" , "An Indian rubber" , "A student's rubber, or the one who uses it" .)