Unrivalled lady about 50 (8)
I believe the answer is:
peerless
'unrivalled' is the definition.
(synonyms)
'lady about 50' is the wordplay.
I cannot really understand how this works, but
'50' could be 'l' (Roman numeral) and 'l' is found in the answer.
'lady' could be 'peeress' (synonyms) and 'peeress' is present in the remaining letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for peerless that I've seen before include "Without equal or lacking lords" , "Without equal, matchless" , "Without equal or match" , "Having no equals" , "Without equal, being without a lord?" .)