Object encountered by one on return (4)
I believe the answer is:
item
'object' is the definition.
(both can mean a thing)
'encountered by one on return' is the wordplay.
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'by' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'on return' says the letters should be written backwards.
'met'+'i'='meti'
'meti' back-to-front is 'ITEM'.
(Other definitions for item that I've seen before include "Article, object" , "One thing on a list" , "Piece in paper" , "newspaper article" , "Individual article or unit" .)