Cool about new punctuation mark (5)
I believe the answer is:
colon
'punctuation mark' is the definition.
(I know that colon is a type of punctuation mark)
'cool about new' is the wordplay.
'about' indicates anagramming the letters.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'cool' with letters rearranged gives 'colo'.
'colo'+'n'='COLON'
(Other definitions for colon that I've seen before include "Stop (less than full)" , "cut short dependency?" , "Punctuate part of the large intestine" , "Stop (on the inside!)" , "Part of body; two dots" .)