Bound to upset us a bit more (4)
I believe the answer is:
sure
'bound to' is the definition.
(synonyms)
'upset us a bit more' is the wordplay.
'upset' says the letters should be written in reverse.
'a bit more' becomes 're' (I can't justify this - if you can you should give a lot more credence to this answer).
'us' back-to-front is 'su'.
'su'+'re'='SURE'
Can you help me to learn more?
(Other definitions for sure that I've seen before include "certainly in the USA" , "Completely certain" , "Agreed" , "Certain, confident" , "Positive" .)