Cool cats, initially in tartan (6)
I believe the answer is:
placid
'cool' is the definition.
(thesaurus)
'cats initially in tartan' is the wordplay.
'initially' says to take the initial letters.
'in' means one lot of letters goes inside another.
'tartan' becomes 'plaid' ('plaid' can be a synonym of 'tartan').
The initial letter of 'cats' is 'c'.
'c' going into 'plaid' is 'PLACID'.
(Other definitions for placid that I've seen before include "Unexcitable" , "Peaceful - composed" , "Calm and still" , "Still - US lake" , "or is it an American one?" .)