I, on the way back, came across a separate article (4)
I believe the answer is:
item
'a separate article' is the definition.
(I've seen this before)
'i on the way back came across' is the wordplay.
'on the way back' says the letters should be written in reverse.
'came across' becomes 'met' (to come across is to meet or encounter).
'met' reversed gives 'tem'.
'i'+'tem'='ITEM'
(Other definitions for item that I've seen before include "Bit of information" , "Two people considered together (informal)" , "Time (anag.)" , "Object in a list" , "Two in a romantic relationship?" .)