Article one got together retrospectively (4)
I believe the answer is:
item
'article' is the definition.
(both can mean an object or thing)
'one got together retrospectively' is the wordplay.
'one' becomes 'i' (Roman numeral).
'got together' becomes 'met' ('meet' can be a synonym of 'get together').
'retrospectively' is a reversal indicator.
'met' in reverse letter order is 'tem'.
'i'+'tem'='ITEM'
(Other definitions for item that I've seen before include "Report" , "Time (anag.)" , "Also no more" , "One thing - two people, perhaps" , "Indivual unit" .)