Ring users with radical compounds, about 50 (9)
I believe the answer is:
wrestlers
'ring users' is the definition.
(people who fight in the ring)
'with radical compounds about 50' is the wordplay.
'with' becomes 'w' (abbreviation).
'radical' becomes 'r' (chemistry abbreviation).
'compounds' becomes 'esters' (ester is a type of chemical compound).
'about' indicates putting letters inside.
'50' becomes 'l' (Roman numeral).
'esters' placed around 'l' is 'estlers'.
'w'+'r'+'estlers'='WRESTLERS'
(Another definition for wrestlers that I've seen is " Sumo types".)