Noblewoman crossing line - it's unparalleled! (8)
I believe the answer is:
peerless
'unparalleled' is the definition.
(thesaurus)
'noblewoman crossing line' is the wordplay.
'noblewoman' becomes 'peeress' ('peeress' can be a synonym of 'noblewoman').
'crossing' means one lot of letters goes inside another.
'line' becomes 'l'.
'peeress' enclosing 'l' is 'PEERLESS'.
'it's' acts as a link.
(Other definitions for peerless that I've seen before include "Unequalled" , "Without equal, nonpareil" , "Unique" , "Unrivalled" , "No Iolanthe performance could be so" .)