Cool about new mark (5)
I believe the answer is:
colon
'mark' is the definition.
(I know that colon is a type of punctuation)
'cool about new' is the wordplay.
'about' indicates anagramming the letters.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'cool' anagrammed gives 'colo'.
'colo'+'n'='COLON'
(Other definitions for colon that I've seen before include "Part of body; some punctuation" , "City at the Caribbean entrance to the Panama Canal" , "Panama port; part of intestine" , "Stop (on the inside!)" , "Stop (less than full)" .)