Article among garments, Roman ones (5)
I believe the answer is:
togas
'roman ones' is the definition.
Although both the answer and definition are plural nouns, I don't see how one could define the other.
'article among garments' is the wordplay.
'article' becomes 'a' (indefinite article).
'among' indicates putting letters inside.
'garments' becomes 'togs' ('tog' can be a synonym of 'garment').
'a' inserted into 'togs' is 'TOGAS'.
(Other definitions for togas that I've seen before include "clothing" , "garments worn by ancient Romans" , "Roman garments" , "What some ancient citizens wore" .)