Upset Burt and Sid? Rubbish! (7)
I believe the answer is:
disturb
'upset' is the definition.
(I know that upset can be written as disturb)
'burt and sid? rubbish' is the wordplay.
'and' is a charade indicator (letters next to each other).
'rubbish' indicates anagramming the letters (a poor quality or rubbish spelling of the letters).
'burt' put after 'sid' is 'sidburt'.
'sidburt' anagrammed gives 'DISTURB'.
(Other definitions for disturb that I've seen before include "Intrude on" , "inconvenience" , "Upset - agitate" , "Upset, confuse" , "Interrupt, annoy" .)