Bound to upset the user (4)
I believe the answer is:
sure
'bound to' is the definition.
(synonyms)
'upset the user' is the wordplay.
'upset' indicates anagramming the letters.
'user' anagrammed gives 'SURE'.
(Other definitions for sure that I've seen before include "of course" , "Confident sort of ruse" , "See 12" , "Completely certain" , "certainly in the USA" .)