Rings one set aside at the beginning (6)
I believe the answer is:
annuli
'rings' is the definition.
(plural of annulus - 'annulus' means a ring)
'one set aside at the beginning' is the wordplay.
'one' becomes 'i' (Roman numeral).
'set aside' becomes 'annul' (to annul is to cancel or set aside).
'at the beginning' says to put letters next to each other (some letters go at the beginning of others).
'i' put after 'annul' is 'ANNULI'.
(Other definitions for annuli that I've seen before include "Areas between two concentric circles" , "Ring-shaped objects" , "round these parts" .)