Denoting one endless line, unparalleled (6)
I believe the answer is:
unique
'unparalleled' is the definition.
(I know that unparalleled can be written as unique)
'denoting one endless line' is the wordplay.
I cannot quite see how this works, but
'one' could be 'un' (northern English dialect form of 'one') and 'un' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for unique that I've seen before include "Sole" , "Singular, without equal" , "Single" , "Nonpareil" , "Unlike anything else" .)