Couple upset one after the other (4)
I believe the answer is:
item
'couple' is the definition.
(I've seen this before)
'upset one after the other' is the wordplay.
'upset' shows that the letters should be reversed in order.
'one' becomes 'me' (a setter might use 'one' to humourously mean 'me').
'after' means one lot of letters go next to another.
'the other' becomes 'it' (old-fashioned UK slang for sex).
'me' written backwards gives 'em'.
'em' after 'it' is 'ITEM'.
(Other definitions for item that I've seen before include "Two in a romantic relationship?" , "date" , "Single thing, poor mite" , "Individual unit in a list" , "Bit of news to broadcast" .)