Unrivalled noblewoman, about fifty (8)
I believe the answer is:
peerless
'unrivalled' is the definition.
(synonyms)
'noblewoman about fifty' is the wordplay.
'noblewoman' becomes 'peeress' (synonyms).
'about' indicates putting letters inside.
'fifty' becomes 'l' (Roman numeral).
'peeress' going around 'l' is 'PEERLESS'.
(Other definitions for peerless that I've seen before include "Unique" , "Outstanding" , "superb" , "Without equal, matchless" , "Beyond compare" .)