Dull piece of music Queen rejected (4)
I believe the answer is:
numb
'dull piece' is the definition.
'numb' can be an answer for 'dull' ('numb' can be a synonym of 'dull'). I am unsure of the 'piece' bit.
'music queen rejected' is the wordplay.
'music' becomes 'b'.
'queen' becomes 'mun' (I can't justify this - if you can you should believe this answer much more).
'rejected' shows that the letters should be reversed in order.
'b'+'mun'='bmun'
'bmun' in reverse letter order is 'NUMB'.
'of' acts as a link.
(Other definitions for numb that I've seen before include "Stinks" , "Insensitive" , "Insensate" , "Dead" , "Deaden" .)