Cool about wearing tartan (6)
I believe the answer is:
placid
'cool' is the definition.
(thesaurus)
'about wearing tartan' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa').
'wearing' indicates putting letters inside.
'tartan' becomes 'plaid' ('plaid' can be a synonym of 'tartan').
'c' going within 'plaid' is 'PLACID'.
(Other definitions for placid that I've seen before include "Unconcerned" , "Composed or serene" , "Tranquil, even-tempered" , "Not ruffled" , "Calm; Lake" .)