Bound to upset us again (4)
I believe the answer is:
sure
'bound to' is the definition.
(synonyms)
'upset us again' is the wordplay.
'upset' shows that the letters should be reversed in order.
'again' becomes 're' (prefix meaning 'again').
'us' reversed gives 'su'.
'su'+'re'='SURE'
(Other definitions for sure that I've seen before include "Certain to be a ruse" , "Undoubtedly true" , "Completely certain" , "Convinced" , "Completely confident that one is right" .)