They meet in the centre of the ring (5)
I believe the answer is:
radii
'they meet in the centre of the ring' is the definition.
The definition suggests an adverb but the answer is not.
'they meet in the centre of the ring' is the wordplay.
'they meet' becomes 'adi' (I can't explain this - if you can you should give a lot more credence to this answer).
'in the' means one lot of letters goes inside another.
'centre of' says to take the centre.
'the ring' becomes 'gloriole' (I have seen 'Ring of light' mean 'gloriole' so perhaps 'ring' could also mean 'gloriole').
The central letters of 'gloriole' are 'ri'.
'adi' going within 'ri' is 'RADII'.
Can you help me to learn more?
(Other definitions for radii that I've seen before include "Half diameters" , "Forearm bones: spokes" , "Spokes of wheel - bones of arm" , "Lines from the centre to the circumference of a circle" , "Lines between the center and circumference of a circle" .)