I encountered upsetting thing (4)
I believe the answer is:
item
'thing' is the definition.
(an item is a thing)
'i encountered upsetting' is the wordplay.
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'upsetting' shows that the letters should be reversed in order.
'met' backwards is 'tem'.
'i'+'tem'='ITEM'
(Other definitions for item that I've seen before include "Loving couple" , "Unit; detail" , "Unit or distinct part" , "Element" , "Individual thing" .)