They happen to upset Steven (6)
I believe the answer is:
events
'they happen' is the definition.
(I've seen this before)
'upset steven' is the wordplay.
'upset' indicates anagramming the letters.
'steven' is an anagram of 'EVENTS'.
'to' is the link.
(Other definitions for events that I've seen before include "races?" , "results" , "Things taking place" , "Noteworthy happenings" , "Important happenings for Steven" .)