Distinguishing mark about Che, the first (6)
I believe the answer is:
cachet
'distinguishing mark' is the definition.
(I've seen this before)
'about che the first' is the wordplay.
'about' becomes 'ca' (circa).
'the first' becomes 't' (1st letter of 'the').
'ca'+'che'+'t'='CACHET'
(Other definitions for cachet that I've seen before include "Official seal - prestige" , "Seal; mark of prestige" , "French prestige or distinguishing mark" , "Mark of approval" , "Official seal on document" .)