Two will reach across the ring (5)
I believe the answer is:
radii
'two will reach across the ring' is the definition.
I can't tell whether this definition defines the answer.
'two will reach across' is the wordplay.
'two' becomes 'II' (Roman numerals).
'will reach' becomes 'rad' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'across' means one lot of letters go next to another (across can mean on).
'ii' after 'rad' is 'RADII'.
Can you help me to learn more?
(Other definitions for radii that I've seen before include "Lines drawn from a circle's centre" , "spoke several times" , "parts for arms" , "Half-diameters" , "Forearm bones: spokes" .)