More dull, rug sent back by Queen (5)
I believe the answer is:
tamer
'more dull' is the definition.
(tame can mean dull or unexciting)
'rug sent back by queen' is the wordplay.
'rug' becomes 'mat' (both are floor coverings).
'sent back' shows that the letters should be reversed in order.
'by' is a charade indicator (letters next to each other).
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'mat' in reverse letter order is 'tam'.
'tam'+'er'='TAMER'
(Other definitions for tamer that I've seen before include "less offensive?" , "Animal trainer" , "Less wild" , "Less arousing, exciting" .)