It upsets me in particular (4)
I believe the answer is:
item
'particular' is the definition.
(I know that particular can be written as item)
'it upsets me' is the wordplay.
'upsets' is a reversal indicator.
'me' in reverse letter order is 'em'.
'it'+'em'='ITEM'
'in' is the link.
(Other definitions for item that I've seen before include "Loving couple" , "Report" , "Unit or distinct part" , "point" , "Article, object" .)