Piece of news one came across on the way back (4)
I believe the answer is:
item
'piece of news' is the definition.
(I've seen this before)
'one came across on the way back' is the wordplay.
'one' becomes 'i' (Roman numeral).
'came across' becomes 'met' ('meet' can be a synonym of 'come across').
'on the way back' says the letters should be written in reverse.
'met' written backwards gives 'tem'.
'i'+'tem'='ITEM'
(Other definitions for item that I've seen before include "Indivual unit" , "Matter" , "Piece of news: unit" , "newspaper article" , "Also no more" .)