Encountered one returning couple (4)
I believe the answer is:
item
'couple' is the definition.
(a romantic couple are 'an item')
'encountered one returning' is the wordplay.
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'one' becomes 'I' (Roman numeral).
'returning' says the letters should be written in reverse.
'met'+'i'='meti'
'meti' back-to-front is 'ITEM'.
(Other definitions for item that I've seen before include "Two people romantically linked" , "Detail (in a list)" , "One thing on a list" , "Object; piece" , "A thing or unit" .)