Curved line is a disgrace (4)
I believe the answer is:
slur
'a disgrace' is the definition.
(I've seen this before)
'curved line' is the wordplay.
I cannot quite see how this works, but
'line' could be 'l' (used when specifying particular lines from a poem) and 'l' is found in the answer.
The remaining letters 'sur' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'is' is the link.
Can you help me to learn more?
(Other definitions for slur that I've seen before include "Slight - incoherence in speech" , "libel" , "Disparagement" , "Slight, insult" , "Pronounce indistinctly or cast abuse on reputation" .)