Pupil, unnamed, goes round boxing rings (6)
I believe the answer is:
annuli
'rings' is the definition.
(plural of annulus - 'annulus' means a ring)
'pupil unnamed goes round boxing' is the wordplay.
'boxing' indicates the answer is hidden within the clue (the solution is contained or boxed up by the clue).
'goes round' shows that the letters should be reversed in order.
'ilunna' can be found hidden inside 'pupil unnamed'.
'ilunna' reversed gives 'ANNULI'.
(Other definitions for annuli that I've seen before include "Areas between two concentric circles" , "round these parts" , "Ring-shaped objects" .)