Detail one came across on the way back (4)
I believe the answer is:
item
'detail' is the definition.
('item' can be a synonym of 'detail')
'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' shows that the letters should be reversed in order.
'met' written backwards gives 'tem'.
'i'+'tem'='ITEM'
(Other definitions for item that I've seen before include "One article" , "Two people widely seen as one?" , "Romantic couple" , "Particular" , "Single article on a list" .)