Couple one encountered upon return (4)
I believe the answer is:
item
'couple' is the definition.
(I've seen this before)
'one encountered upon return' is the wordplay.
'one' becomes 'I' (Roman numeral).
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'upon' is a charade indicator (letters next to each other).
'return' says the letters should be written backwards.
'i' after 'met' is 'meti'.
'meti' in reverse letter order is 'ITEM'.
(Other definitions for item that I've seen before include "See couple" , "Boyfriend and girlfriend" , "Book-keeping entry" , "Separate article" , "Element" .)