Dull, ordinary and unacceptable in sumo wrestling (10)
I believe the answer is:
monotonous
'dull' is the definition.
(similar in meaning)
'ordinary and unacceptable in sumo wrestling' is the wordplay.
'ordinary' becomes 'O' (abbreviation e.g. O-Levels).
'and' is a charade indicator (letters next to each other).
'unacceptable' becomes 'not on' (something 'not on' is unacceptable or frowned upon).
'in' indicates putting letters inside.
'wrestling' indicates anagramming the letters.
'o' after 'noton' is 'notono'.
'sumo' anagrammed gives 'mous'.
'notono' placed within 'mous' is 'MONOTONOUS'.
(Other definitions for monotonous that I've seen before include "Dull, boring" , "Humdrum" .)