Oscar presumably off to interrupt my dull routine (8)
I believe the answer is:
monotony
'dull routine' is the definition.
(monotonous routine)
'oscar presumably off to interrupt my' is the wordplay.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'presumably off' becomes 'not on'.
'to interrupt' indicates putting letters inside.
'o'+'noton'='onoton'
'onoton' going into 'my' is 'MONOTONY'.
(Other definitions for monotony that I've seen before include "fixed routine" , "Lack of excitement" , "Sameness" , "boring routine" , "Tedious lack of variety" .)