Make a note of unparalleled achievement (6)
I believe the answer is:
record
I believe this clue is a double definition.
'make a note' is the first definition.
(thesaurus)
'unparalleled achievement' is the second definition.
'of' acts as a link.
(Other definitions for record that I've seen before include "Best known performance in sport" , "Set down in writing" , "Register - fastest time" , "Write down; music disc" , "Log; disc" .)