Certain to get us upset again (4)
I believe the answer is:
sure
'certain' is the definition.
(I know that certain can be written as sure)
'get us upset again' is the wordplay.
'get' means one lot of letters go next to another.
'upset' says the letters should be written backwards.
'again' becomes 're' (prefix meaning 'again').
'us' back-to-front is 'su'.
'su' put next to 're' is 'SURE'.
'to' acts as a link.
(Other definitions for sure that I've seen before include "Confident sort of ruse" , "Definite" , "Undoubtedly true" , "The old sewer" , "Agreed" .)